Porous catalyst-support shaped body

ABSTRACT

A porous shaped catalyst support body comprising at least 85% by weight of alpha-alumina, wherein the support has a total pore volume in the range from 0.5 to 2.0 mL/g as determined by mercury porosimetry, and a pore structure characterized by a geometric tortuosity τ in the range from 1.0 to 2.0; and an effective diffusion parameter η in the range from 0.060 to 1.0; wherein geometric tortuosity τ and effective diffusion parameter η are determined by image analysis algorithms from computer-assisted 3D reconstructions of focused ion beam scanning electron microscope analyses. The structure of the support has a high total pore volume, such that impregnation with a large amount of silver is possible, while the surface area is kept sufficiently high in order to assure optimal dispersion of the catalytically active species, especially metal species. The support has a pore structure that leads to a maximum rate of mass transfer within the support. The invention also relates to a shaped catalyst body for preparation of ethylene oxide by gas phase oxidation of ethylene, comprising at least 15% by weight of silver, based on the total weight of the catalyst, deposited on a porous shaped catalyst support body as described above. The invention further relates to a process for producing the shaped catalyst body, in which a) a porous shaped catalyst support body as described above is impregnated with a silver impregnation solution, preferably under reduced pressure; and the impregnated porous shaped catalyst support body is optionally subjected to drying; and b) the impregnated porous shaped catalyst support body is subjected to a heat treatment; wherein steps a) and b) are optionally repeated. The invention also relates to a process for preparing ethylene oxide by gas phase oxidation of ethylene, comprising the reaction of ethylene and oxygen in the presence of a shaped catalyst body according to claim 11.

DESCRIPTION

The present invention relates to a porous shaped catalyst support body, to a shaped catalyst body for preparation of ethylene oxide by gas phase oxidation of ethylene, to a process for preparing the shaped catalyst body and to a process for preparing ethylene oxide by gas phase oxidation of ethylene.

Alumina (Al₂O₃) is ubiquitous in supports and/or catalysts for many heterogeneous catalytic processes. Some of these catalytic processes take place under conditions with high temperature, high pressure and/or high steam pressure. It is known that alumina has a number of crystalline phases such as alpha-alumina (often referred to as α-alumina or α-Al₂O₃), gamma-alumina (often referred to as γ-alumina or γ-Al₂O₃), and a number of alumina polymorphs. alpha-Alumina is the most stable at high temperatures, but has the lowest surface area.

gamma-Alumina has a very high surface area. It is generally assumed that this is attributable to the fact that the aluminum oxide molecules are in a crystalline structure which is not very tightly packed. gamma-Alumina is one of what are called the activated aluminas or transition aluminas, since it is one of a number of aluminas that can be converted to various polymorphs. Regrettably, the atomic structure collapses when gamma-alumina is heated to high temperatures, and so the surface area is greatly reduced. The most dense crystalline form of the aluminas is alpha-alumina.

Ethylene oxide is produced in large volumes and is used mainly as intermediate in the preparation of various industrial chemicals. In the industrial oxidation of ethylene to ethylene oxide, typically heterogeneous catalysts are used, comprising silver deposited on a porous support. For performance of the heterogeneously catalyzed gas phase oxidation, in general, a mixture of an oxygenous gas, for example air or pure oxygen, and ethylene is directed through a multitude of tubes disposed in a reactor in which there is a packing of shaped catalyst bodies.

Catalyst performance is typically characterized by selectivity, activity, longevity of catalyst selectivity and activity, and mechanical stability. Selectivity is the molar proportion of the olefin converted that results in the desired olefin oxide. Even small improvements in selectivity and maintenance of selectivity over a prolonged period of time bring enormous advantages in relation to process efficiency.

In order that the internal surface areas of a porous supported catalyst can be utilized effectively, the feed gases must diffuse through the pores in order to reach the inner surface areas, and the reaction products must diffuse away from these surfaces and out of the catalyst body. In a process for preparing ethylene oxide by gas phase oxidation of ethylene, the diffusion of ethylene oxide molecules out of the catalyst bodies may be accompanied by unwanted further reactions that are caused by the catalyst, such as isomerization to acetaldehyde, followed by full combustion to carbon dioxide, which reduces the overall selectivity of the process. The average dwell times of the molecules in the pores and hence the degree to which unwanted further reactions occur are influenced by the pore structure of the catalyst.

Catalytic performance is thus influenced by the pore structure of the catalyst, which is determined essentially by the pore structure of the catalyst support. The term “pore structure” is understood to mean the arrangement of cavities within the support matrix, including size, size distribution, shape and interconnectivity of the pores. It can be characterized by various methods such as mercury porosimetry, nitrogen physisorption or tomography methods. H. Giesche, “Mercury Porosimetry: A General (Practical) Overview”, Part. Part. Syst. Charact. 23 (2006), 9-19, imparts helpful insights in relation to mercury porosimetry.

EP 2 617 489 A1 describes a catalyst support in which at least 80% of the pore volume is present in pores having diameters in the range from 0.1 to 10 µm, and at least 80% of the pore volume in pores having diameters in the range from 0.1 to 10 µm is present in pores having diameters in the range from 0.3 to 10 µm.

WO 03/072244 A1 and WO 03/072246 A1 each describe a catalyst support in which at least 70% of the pore volume is present in pores having diameters of 0.2 to 10 µm, and pores having diameters between 0.2 to 10 µm represent a volume of at least 0.27 mL/g of the support.

EP 1 927 398 A1 describes a catalyst support having a pore size distribution having at least two maxima in the range from 0.01 to 100 µm, where at least one of these maxima is in the range from 0.01 to 1.0 µm.

EP 3 639 923 A1 describes a shaped catalyst body having a multimodal pore size distribution having a maximum in the range from 0.1 to 3.0 µm and a maximum in the range from 8.0 to 100 µm, where at least 40% of the total pore volume of the shaped catalyst body comes from pores having a diameter in the range from 0.1 to 3.0 µm.

WO 2021/038027 A1 describes a catalyst for preparation of ethylene oxide using a porous alumina support having a foam-like structure.

However, the characterization of catalyst supports by their pore size distribution, for example by the assignment of pore size ranges to fractions of the total pore volume, does not fully characterize the morphology and usability of porous catalyst supports. In order to correctly determine the characteristics of mass transfer through the pores of porous solids, it is necessary to determine the structural parameters of a material, such as porosity, pore tortuosity and pore constriction. In particular, pore size distributions cannot show how many bends, convolutions, dead ends and local changes in cross section, for example constrictions, are possessed by the channels within porous media.

US 2016/0354760 A1 relates to a porous body comprising at least 80% alpha-alumina and having a pore volume of 0.3 to 1.2 mL/g, a surface area of 0.3 to 3.0 m²/g and a pore architecture having a tortuosity of 7 or less, a constriction of 4 or less and/or a permeability of 30 mdarcys or more. Tortuosity, constriction and permeability were calculated from Hg intrusion data. For example, tortuosity, ξ, was calculated from the following equation, where D_(avg) is the weighted average pore size, k is the permeability, ρ is the true density and I_(tot) is the total specific intrusion volume:

$\xi = \sqrt{\frac{D_{avg}^{2}}{4 \cdot 24k\left( {1 - \rho I_{tot}} \right)}}$

Constriction σ was calculated by the following equation, where ξ is the tortuosity and τ is the tortuosity factor:

$\sigma = \frac{\xi}{\tau}$

The illustrative supports from US 2016/0354760 A1 have a constriction σ in the range from 1.6 to 5.3.

There is still a considerable need to improve the properties of a supported catalyst by optimization of the structure of the alumina-based support. The structure of the support should have a high total pore volume, such that impregnation with a large amount of silver is possible, while the surface area should be kept sufficiently high in order to assure optimal dispersion of the catalytically active species, especially metal species. A pore structure that leads to a maximum rate of mass transfer within the support is also desirable in order to minimize the average pore dwell times of the reactant and product molecules and to limit the extent to which primary reaction products such as ethylene oxide enter into unwanted secondary reactions while they are diffusing through the pores of a supported catalyst. Furthermore, the supported shaped catalyst body, in spite of the described requirements of a high pore volume and an adequate pore structure for high rates of mass transfer within the pores, should have a high density in the packed tube and high mechanical strength.

The invention relates to a porous shaped catalyst support body comprising at least 85% by weight of alpha-alumina, wherein the support has a total pore volume in the range from 0.5 to 2.0 mL/g as determined by mercury porosimetry, and a pore structure characterized by

-   a geometric tortuosity τ in the range from 1.0 to 2.0; and -   an effective diffusion parameter η in the range from 0.060 to 1.0;

wherein geometric tortuosity τ and effective diffusion parameter η are determined by image analysis algorithms from computer-assisted 3D reconstructions of focused ion beam scanning electron microscope (FIB-SEM) analyses.

According to the invention, geometric tortuosity τ and effective diffusion parameter η are determined by image analysis algorithms from computer-assisted 3D reconstructions of focused ion beam scanning electron microscope (FIB-SEM) analyses. It is assumed that this methodology enables a more meaningful determination of τ and η than phenomenological methods such as mercury porosimetry. Mercury porosimetry is based on static measurements, i.e. equilibrium measurements, that are not sufficiently meaningful for the structural parameters of relevance in dynamic transport processes. The methodology for determination of geometric tortuosity τ and effective diffusion parameter η is elucidated hereinafter.

The kinetics of diffusive mass transfer within spatially bounded porous bodies depends on various considerations, including (i) intrinsic transport parameters of the fluid to be transported, for example the molecular diffusion coefficient under the operating conditions, and (ii) the porous structure of the solid that determines the possible transport pathways.

Diffusive molecular transport processes in the coherent gas phase, i.e. without restrictions of the transport pathways by a porous solid, are driven by spatial gradients in fugacity (concentration in the case of ideal gases) of chemical compounds and are typically described by Fick’s first law:

$\begin{matrix} {J = - \mathfrak{D}_{b} \cdot \nabla\text{φ}} & \text{­­­Eq. 1} \end{matrix}$

where Jrepresents the flow rate of the molecular compound, ∇φ denotes the gradient of fugacity of the compound that acts as the driving force for transport, and D_(b) is the coefficient of molecular diffusion. In the case of diffusion transport that takes place within a porous solid, for example a solid catalyst, an effective coefficient of diffusion D_(eff) is defined in order to correct the restrictions of the transport pathways that are caused by the porous structure of the solid by comparison with unhindered transport in the coherent (bulk) gas phase. According to a large portion of the scientific literature, this effective coefficient of diffusion can be defined as (J.J. Kane et al., Carbon 136 (2018) 369-379):

$\begin{matrix} {\mathcal{D}_{eff} = \frac{\mathcal{D}_{b} \cdot \varepsilon}{\tau} \cdot \left( \frac{\sigma}{\tau} \right)} & \text{­­­Eq. 2} \end{matrix}$

where ε represents the porosity of the solid, τ is the pore tortuosity of the solid and σ is the pore constriction of the solid. Tortuosity is an intrinsic property of a porous solid, which is typically defined as the ratio of the possible flow pathway length through the pore structure relative to the straight distance between the ends of that flow pathway. On the basis of its definition, τ assumes values of not less than 1.

Pore constriction σ is a further intrinsic property of a porous solid, which is typically defined as the ratio between the cross-sectional area of a flow pathway at the narrowest point, i.e. at the point where the cross-sectional area is at a minimum, and the cross-sectional area of the flow pathway at the broadest point, i.e. at the point where the cross-sectional area is at a maximum. On the basis of this commonly accepted definition which is adopted here, σ assumes values in the range from 0 to 1, where 0 is the constriction value for a pore blocked at a point along the flow pathway, and 1 is the constriction value for a pore having constant cross-sectional area along the total flow pathway, for example a cylindrical pore.

Therefore, an effective diffusion parameter (η) defined as:

$\begin{matrix} {\eta = \left( \frac{\mathcal{D}_{eff}}{\mathcal{D}_{b}} \right) = \frac{\varepsilon \cdot \sigma}{\tau^{2}}} & \text{­­­Eq. 3} \end{matrix}$

describes the extent to which molecular diffusion through a given porous solid is restricted in relation to molecular diffusion in the continuous gas phase under otherwise identical conditions of temperature, pressure and fugacity gradients. On the basis of its definition, the parameter η assumes values in the range from 0 to 1. Solid porous bodies having a higher value of η have higher rates of pore diffusion mass transfer.

Equation 4 below establishes a relationship between porosity, pore diameter and specific surface area of a porous solid:

$\begin{matrix} {d = \frac{4 \cdot \varepsilon}{A \cdot f \cdot \left( {1 - \varepsilon} \right) \cdot \rho_{sk}}} & \text{­­­Eq. 4} \end{matrix}$

where A is the mass-specific surface area, f is an arbitrary shape factor that takes account of the variances of the real pore cross section from the cylindrical pore cross section, and ρ_(sk) defines the density of the solid framework of the porous solid that defines the pores.

Porosity is the proportion of the total volume of a porous solid that corresponds to the cavities. In relation to the determination of the transport of fluids through porous solids, the definition is often limited to the cavity volume that percolates to the outer surface of the solid (is connected).

Porosity can be determined by various experimental methods known in the specialist field, which include:

-   (i) gas physisorption, -   (ii) mercury porosimetry, -   (iii) water thermo- or cryoporometry and -   (iv) direct quantification with the aid of image analysis algorithms     from computer-assisted 3D reconstructions of the pore structure of     solids.

3D reconstructions of the pore structure of solids can be obtained by a number of tomography imaging methods. These include x-ray computed microtomography (micro-CT), electron tomography (ET), focused ion beam scanning electron microscopy (FIB-SEM) tomography and nuclear spin resonance (NMR) tomography. The (raw) tomograms recorded typically consist of a collection of uniform parallelepipedic (often cubic) information volumes or voxels that collectively represent the structure of the material depicted. Each voxel is assigned the x,y,z coordinates that correspond to its geometric center in 3D space, and a grayscale value (e.g. 0 for pure black to 255 for pure white) that comprises information about the composition of the material depicted at the specific voxel position. In order to assure that the depicted volume is representative and that the structural parameters ascertained therefrom are highly accurate, it is preferable to conduct 3D reconstructions on a sample volume having a side length at least 10 times greater than the median pore diameter value of the material, preferably at least 15 times greater than the median pore diameter value of the material, and even more preferably at least 20 times greater than the median pore diameter value of the material. In addition, it is preferable in this connection to use imaging parameters that lead to elemental information units (voxels) having side lengths at least 10 times less than the median pore diameter value of the material, preferably at least 20 times less than the median pore diameter value of the material and more preferably at least 50 times less than the median pore diameter value of the material.

In order to extract structural information about the porous solids from the corresponding tomograms, image analysis methods are typically used. Image analysis typically first includes segmentation of the raw tomograms, i.e. assignment of all voxels in the reconstructed tomograms to different phases, i.e. a phase having hollow pores and a phase having solid pore walls, on the basis of the individual grayscale contrast value of the voxel.

Segmentation methods known in the prior art are based, for example, on watershed algorithms, as described in E. Dougherty, editor, Mathematical morphology in image processing, chapter 12, pages 433-481, Marcel Dekker, 1993, and A. Bieniek, A. Moga, Pattern Recognition (2000) 33, Issue 6, 907-916, or on contrast mask region-based convolutional neural network algorithms, as described in He, K., et al.,

Proceedings of the IEEE International Conference on Computer Vision (2017) 2961-2969. The porosity of a solid can be calculated as the ratio of total volume for voxels that are assigned to the phase of the hollow pores to the total volume of all voxels in a tomogram.

In order to facilitate segmentation of tomograms and to increase accuracy, i.e. to improve delimitation of interfaces between different phases in a single tomogram, various approaches are taken in the specialist field in order to amplify differences in contrast at these boundaries without altering the structure of the sample material. Interfaces between different phases in a single tomogram are, for example, the interface between the phases of the solid framework and the hollow pores in a porous material.

A known approach is to deposit a heavy metal contrast agent, for example a compound of tungsten (W), rhenium (Re) or osmium (Os), as an overlay on the surface of the solid base skeleton. Another known approach is the fine distribution of a metal or an alloy in the form of metal nanoparticles on the surface of the solid base skeleton. For the present purposes, we propose a simplified and reliable method comprising the FIB-SEM analysis of a silver-laden epoxidation catalyst body, i.e. an alumina support with silver deposited thereon, and the mathematical removal of the silver during the image analysis, in order to examine the pore structure of the underlying alumina support.

In FIB-SEM tomography, the sample to be examined is typically infiltrated with a resin, for example an epoxy resin, in order to remove background signals of underlying layers during the SEM imaging. Such a resin infiltration thus assists image segmentation.

FIGS. 1 to 3 show illustrative surface-rendered three-dimensional FIB-SEM tomograms of a 10 µm x 10 µm x 10 µm cubic volume component of a porous metal-on-Al₂O₃ support catalyst that were obtained after segmentation and analysis thereof.

Pore tortuosity is an important topological parameter for description of porous solids. In formal terms, this parameter describes how the permissible flow pathways differ from the straight line on account of the fact that transport is limited to a porous solid. There are multiple known methods in the specialist field that can be employed for determination of pore tortuosity for porous solids. As described in Yang, K., et al., Transp. Porous Media (2019) 1-19, the magnitude of the tortuosity value depends on the protocol which is used for determination thereof. The methods known in the specialist field for determination of the tortuosity of porous solids include the following:

-   (i) Diffusive tortuosity is determined by measuring the diffusivity     of an unreactive compound that diffuses through porous solids (Van     Brakel, J., Heertjes, P., Int. J. Heat Mass Transf. (1974) 17 (9),     1093-1103); -   (ii) Electrical tortuosity is determined by measuring the effective     electrical conductivity of an electrolyte fluid surrounded by a     porous solid (Landesfeind, J., et al., J. Electrochem. Soc. (2016)     163 (7), A1373-A1387); -   (iii) Hydraulic tortuosity is determined by measuring the     permeability of a fluid which is transported through porous solids     under a pressure gradient (Clennell, M.B., Geol. Soc. Lond., Spec.     Publ. (1997) 122 (1), 299-344); -   (iv) Geometric tortuosity is defined as the ratio between the     effective transport pathway length (geodesic length) and the     straight line (Euclidean length) in the direction of macroscopic     flow, and can be determined with the aid of image analysis     algorithms that are applied to computer-assisted 3D reconstructions     of the pore structure of solids, for example segmented tomograms.

The known algorithms for determination of geometric tortuosity from computer-assisted 3D reconstructions of the pore structure of solids include direct shortest path search methods, as described, for example, in Stenzel, O., et al. AICHE J. (2016) 62 (5),1834-1843 and Cecen, A., et al., J. Electrochem. Soc. (2012) 159 (3), B299-B307, skeleton shortest path search methods as described in Lindquist, W.B., et al., J. Geophys. Res. Solid Earth (1996) 101 (B4), 8297-8310 and Al-Raoush, R.I., Madhoun, I.T., Powder Technol. (2017) 320, 99-107, fast marching methods as described in Hassouna, M.S., Farag, A.A., IEEE Trans. Pattern Anal. Mach. Intell. (2007) 29 (9), 1563-1574 and Jorgensen, P.S., et al., J. Microsc. (2011) 244 (1), 45-58, and pore centroid methods as described in Gostovic, D., Electrochem. Solid-State Lett. (2007) 10 (12), B214-B217 and Smith, J., et al., Solid State Ionics (2009) 180 (1), 90-98.

Pore constriction is a further important topological parameter for description of porous solids. In formal terms, this parameter describes changes in cross-sectional area through transport pathways. Constriction is typically defined as the ratio of the cross-sectional area for the narrowest sections (necks) to the cross-sectional area of the broadest sections (pores) along a flow pathway. According to this definition, which is adopted here, pore constriction assumes values in the range from 0 to 1. Alternative definitions can be found in the literature, as discussed, for example, in Holzer, L.; et al. J Mater Sci (2013) 48:2934-2952. In another frequently used definition, Petersen defines construction as the ratio of the cross-sectional area for the broadest section (pore) to the cross-sectional area of the narrowest section (neck) along a flow pathway (Petersen, E.E. (1958), Diffusion in a Pore of Varying Cross Section. AlChE J., 4: 343-345). According to this definition, which was not adopted here, constriction assumes values greater than or equal to 1.

Information about the narrowest pore necks and the broadest pore segments along the infiltration pathways can be inferred with the aid of methods known in the specialist field, such as mercury intrusion at a constant rate, as described in Gao, H., Li, T. & Yang, L., J Petrol Explor Prod Technol 6, 309-318 (2016). Alternatively, the pore and neck sizes can be determined with the aid of image analysis algorithms that are applied to computer-assisted 3D reconstructions of the pore structure of solids.

There are multiple known algorithms in the specialist field that serve to ascertain information about the neck and pore diameters. In general, the basis of these algorithms is that the 3D pore structure of a solid is first modelled with the aid of a pore network model (PNM). A PNM is a virtual representation of the porosity of the solid, which consists of typically spherical pore bodies and cylindrical pore constrictions of different size that are connected to one another in space as required in order to simulate all local geometric and topological properties of the real pore system in the solid.

There are various methods of constructing a PNM from 3D reconstructions of the pore structure of solids, for example segmented tomograms. These include grain-based models, as described in Bryant, S.L., King, P.R., Mellor, D.W., Transp. Porous Media (1993) 11 (1), 53-70, and Pilotti, M. Transp. Porous Media (2000)41 (3), 359-364, medial axis algorithms as described in Lindquist, W.B., et al., J. Geophys. Res. Solid Earth (2000) 105 (B9), 21509-21527 and Jiang, Z., et al., Water Resour. Res. (2007) 43 (12), and maximum ball algorithms as described in Al-Kharusi, A.S., Blunt, M.J., J. Pet. Sci. Eng. (2007) 56 (4), 219-231 and Dong, H., Blunt, M.J., Phys. Rev. E (2009) 80 (3), 036307.

There are various available software packages with which the person skilled in the art can determine structural parameters such as porosity, tortuosity and constriction by application of the abovementioned and alternative algorithms to 3D tomography reconstructions of the pore structure of solids. These software packages include FIJI-ImageJ®, MATLAB®, AvizoFire®, GeoDict® and available plug-ins and extensions of function for the purpose.

It is apparent from equation 3 that an increase in porosity ε or in pore constriction σ or in both parameters and a decrease in pore tortuosity τ increases the effective coefficient of diffusion through porous solids and hence increases the rate of diffusive mass transfer through the pores.

As discussed, geometric tortuosity τ and effective diffusion parameter η of the porous shaped catalyst support body are determined by image analysis algorithms from computer-assisted 3D reconstructions of focused ion beam scanning electron microscope (FIB-SEM) analyses. More specifically, pore tortuosity is determined by image analysis of a segmented FIB-SEM tomogram for a catalyst that has been obtained by dispersing silver on the support, applying a centroid path algorithm as described in Gostovic, D. et al., Journal of the American Ceramic Society (2011) 94: 620-627, to the set of tomogram voxels that correspond to the pores. The effective diffusion parameter η is determined as the porosity of the material ε multiplied by the pore constriction of the material σ multiplied by τ ⁻²; where the porosity of the material is determined as the proportion of total voxels corresponding to pores in the segmented tomogram; where the pore constriction is determined as the square of the ratio between the average diameter for all necks to the average diameter for all pores in the pore network model of the material; where the pore network model of the material is determined by applying a computational algorithm that combines a chamfer distance transformation in 3D, a watershed operation and a numerical reconstruction to the collection of voxels corresponding to the pores; where the algorithm is adjusted such that it considers connected voxels to be those that have at least one common vertex, and the contrast factor marker of H maxima is adjusted to 2, as described in E. Bretagne (2018) Mineralogical Limitations for X-Ray Tomography of Crystalline Cumulate Rocks, Durham University, and implemented in Avizo® 2020.1-XPore (ThermoScientific).

The porous shaped catalyst support body has a geometric tortuosity τ in the range from 1.0 to 2.0. The porous shaped catalyst support body preferably has a geometric tortuosity τ in the range from 1.0 to 1.75, more preferably in the range from 1.0 to 1.50, especially in the range from 1.0 to 1.30. The porous shaped catalyst support body usually has a geometric tortuosity τ of at least 1.05, or at least 1.1.

The porous shaped catalyst support body has an effective diffusion parameter η in the range from 0.060 to 1.0, preferably in the range from 0.065 to 1.0, especially in the range from 0.070 to 1.0. The porous shaped catalyst support body usually has an effective diffusion parameter η of 0.8 or less, or 0.5 or less, or 0.2 or less.

The porous shaped catalyst support body has a total pore volume in the range from 0.5 to 2.0 mL/g as determined by mercury porosimetry. The porous shaped catalyst support body preferably has a total pore volume in the range from 0.5 to 1.2 mL/g, more preferably in the range from 0.5 to 1.0 mL/g, especially in the range from 0.5 to 0.8 mL/g. Lower total pore volumes can lead to a lower rate of absorption of the metal impregnation solution and hence to lower catalyst activity. Higher total pore volumes can lead to lower densities in the packed tube and hence in turn to a lower catalyst activity.

Total pore volume is determined by mercury porosimetry. Mercury porosimetry is conducted by exerting controlled pressure on a sample immersed in mercury. On account of the high contact angle of mercury, an external pressure is required for the mercury to be able to penetrate into the pores of a material. The level of pressure required for penetration into the pores is inversely proportional to the size of the pores. The larger the pore, the lower the pressure required for penetration into the pore. A mercury porosimeter uses the pressure-to-intrusion data obtained to ascertain volume and pore size distributions using the Washburn equation.

Mercury porosimetry can be conducted with an AutoPore V 9600 mercury porosimeter from Micrometrics (contact angle 140 degrees, Hg surface tension 485 dyn/cm, maximum head pressure 61000 psia). Porosity is determined here to DIN 66133, unless stated otherwise.

The porous shaped catalyst support body preferably has a density in the packed tube of more than 450 g/L. Density in the packed tube is understood to mean the density per liter of a support-packed cylindrical tube having an internal diameter of 39 mm. Density in the packed tube can be determined by the method described below.

The porous shaped catalyst support body preferably has a density in the packed tube in the range from 450 g/L to 1000 g/L, preferably in the range from 480 g/L to 800 g/L, more preferably in the range from 500 g/L to 700 g/L, especially in the range from 520 g/L to 650 g/L. Lower densities in the packed tube lead to reduced catalyst activity. Higher densities in the packed tube can lead to undesirably high catalyst consumption per unit reactor volume, or to a disadvantageously high pressure drop, which leads to elevated energy consumption in processes that are operated in gas recycling mode, for example typical ethylene oxide processes.

The porous shaped catalyst support body can be obtained by a process in which

-   i) a precursor material is provided comprising, based on the content     of inorganic solids,     -   at least 50% by weight of a transition alumina having a loose         bulk density of not more than 600 g/L, a pore volume of at least         0.6 mL/g and an average pore diameter of at least 15 nm; and     -   not more than 30% by weight of an alumina hydrate; -   ii) the precursor material is shaped to shaped bodies; and -   iii) the shaped bodies are calcined to obtain the porous shaped     catalyst support body.

The alpha-alumina formed from transition alumina has a vermicular structure, i.e. no clearly defined particle structure and extended porosity. It generally has a much finer crystal size than preformed alpha-alumina particles without internal porosity that were used for production of catalyst supports according to the prior art. It is assumed that this leads to a porous matrix having channels that have fewer bends and convolutions.

The precursor material, based on the inorganic solids content, comprises at least 50% by weight of a transition alumina. Preferably, the precursor material, based on the inorganic solids content, comprises at least 60% by weight, more preferably at least 70% by weight, of the transition alumina, such as at least 80% by weight or at least 90% by weight, especially 95% to 100% by weight.

The expression “transition alumina” is understood to mean an alumina comprising a metastable alumina phase, such as a gamma-, delta-, eta-, theta-, kappa- or chi-alumina phase. The transition alumina preferably comprises at least 80% by weight, preferably at least 90% by weight, more preferably at least 95% by weight, such as 95% to 100% by weight, of a phase selected from gamma-alumina, delta-alumina and/or theta-alumina, based on the total weight of the transition alumina.

The transition alumina is typically in the form of a powder. Transition aluminas are commercially available and can be obtained by thermal dehydration of hydrated aluminum compounds, especially aluminum hydroxides and aluminum oxyhydroxides. Suitable hydrated aluminum compounds comprise naturally occurring and synthetic compounds, such as aluminum trihydroxides (Al(OH)₃) such as gibbsite, bayerite and nordstrandite, or aluminum oxymonohydroxides (AIOOH) such as boehmite, pseudoboehmite and diaspore.

The stepwise dehydration of hydrated aluminum compounds results in lattice rearrangements. For example, boehmite can be converted at about 450° C. to gamma-alumina, gamma-alumina at about 750° C. to delta-alumina, and delta-alumina at about 1000° C. to theta-alumina. When heated to about 1000° C., transition aluminas are converted to alpha-alumina.

It is assumed that the morphological properties of the transition aluminas thus obtained depend primarily on the morphological properties of the hydrated aluminum compounds from which they have been prepared. It is accordingly stated in Busca, “The Surface of Transitional Aluminas: A Critical Review”, Catalysis Today, 226 (2014), 2-13, that aluminas derived from different pseudoboehmites have different pore volumes and pore size distributions, even though the pseudoboehmites have similar surface areas (160 ~ 200 m²/g).

In a preferred embodiment, the transition alumina comprises non-platelet-shaped crystals. The term “non-platelet-shaped” refers to any shape other than platelet-shaped, for example elongated shapes such as rods or needles, or shapes having approximately the same dimensions in all three spatial directions. In a preferred embodiment, the transition alumina comprises non-platelet-shaped crystals, for example rod-shaped crystals, as described, for example, in WO 2010/068332 A1, or block-shaped crystals, as described, for example, in Busca, “The Surface of Transitional Aluminas: A Critical Review”, Catalysis Today, 226 (2014), 2-13; see FIGS. 2 c, 2 d and 2 e by comparison with FIGS. 2 a, 2 b and 2 f . Preferably, the average crystal size of the transition alumina is at least 5 nm, more preferably at least 7 nm, especially at least 10 nm, as determined by the Scherrer equation from XRD patterns.

Various synthetic methods of obtaining crystalline boehmitic alumina having high pore volume and high surface area coupled with high thermal stability are known, for example from WO 00/09445 A2, WO 01/02297 A2, WO 2005/014482 A2 and WO 2016/022709 A1. WO 2016/022709 A1 describes, for example, boehmitic aluminas having an average pore diameter of 115 to 166 Å, a bulk density of 250 to 350 kg/m³ and a pore volume of 0.8 to 1.1 m³/g, prepared by precipitation of basic aluminum salts with acidic aluminum oxide salts at controlled pH and temperature. Particularly suitable transition aluminas are those that are prepared by thermal treatment of these boehmitic aluminas and have the properties defined in the present claims.

Prior to the heat treatment, the hydrated aluminum compounds may be washed, for example with demineralized water, in order to reduce the level of impurities and make it possible to obtain a transition alumina of high purity. For example, crystalline boehmite that has been obtained from gibbsite by a hydrothermal method according to Chen et al., J. Solid State Chem., 265 (2018), 237 to 243, is preferably washed prior to heat treatment.

Transition aluminas of high purity are preferable in order to limit the content of impurities such as sodium or silicon in the catalyst support. Transition aluminas of high purity can be obtained, for example, by what is called the Ziegler method, in some cases also referred to as the ALFOL method, and variants thereof, as described in Busca, “The Surface of Transitional Aluminas: A Critical Review”, in Catalysis Today, 226 (2014), 2-13. Other methods based on the precipitation of aluminates such as sodium aluminate tend to result in transition aluminas having relatively large amounts of impurities, such as sodium.

The transition aluminas used in the present invention preferably have a total content of alkali metals, e.g. sodium and potassium, of not more than 1500 ppm, more preferably not more than 600 ppm and especially 10 ppm to 200 ppm, based on the total weight of the transition alumina. There are various known washing methods that enable the reduction in the alkali metal content of the transition alumina and/or the catalyst support obtained therefrom. The washing may comprise washing with a base, an acid, water or other liquids.

US 2,411,807 A states that the sodium oxide content in precipitated aluminas can be reduced by washing with a solution comprising hydrofluoric acid and another acid. WO 03/086624 A1 describes a support pretreatment with an aqueous lithium salt solution in order to remove sodium ions from the surface of a support. US 3,859,426 A describes the purification of refractory oxides such as aluminum oxide and zirconium dioxide by repeated rinsing with hot deionized water. WO 2019/039930 describes a method of purifying aluminum oxide, in which metal impurities are removed by extraction with an alcohol.

As well as alkali metals, preference is also given to controlling the amounts of other naturally occurring impurities.

The transition aluminas used in the present invention preferably have a total content of alkaline earth metals, such as calcium and magnesium, of not more than 2000 ppm, more preferably of not more than 600 ppm and especially of not more than 400 ppm, based on the total weight of the transition alumina.

The transition aluminas used in the present invention preferably have a content of silicon of not more than 10000 ppm, more preferably of not more than 2000 ppm and especially of not more than 700 ppm, based on the total weight of the transition alumina.

The transition aluminas used in the present invention preferably have a content of iron of not more than 1000 ppm, more preferably of not more than 600 ppm and especially of not more than 300 ppm, based on the total weight of the transition alumina.

The transition aluminas used in the present invention preferably have a content of metals other than those mentioned above, such as titanium, zinc, zirconium and lanthanum, of not more than 1000 ppm, more preferably of not more than 400 ppm and especially of not more than 100 ppm, based on the total weight of the transition alumina.

The transition alumina has a loose bulk density of not more than 600 g/L. The term “loose bulk density” is understood to mean density “on loose introduction” or “on free-flowing introduction”. “Loose bulk density” thus differs from “tapped density”, in the case of which a defined sequence of mechanical impacts is employed and a higher density is typically achieved. Loose bulk density can be determined by pouring the transition alumina into a measuring cylinder, appropriately via a funnel, ensuring that the measuring cylinder is not moved or agitated. The volume and weight of the transition alumina are determined. Bulk density is determined by dividing the weight in grams by the volume in liters.

A low loose bulk density may indicate a high porosity and a high surface area. The transition alumina preferably has a loose bulk density in the range from 50 to 600 g/L, preferably in the range from 100 to 550 g/L, more preferably 150 to 500 g/L, especially 200 to 500 g/L or 200 to 450 g/L.

The transition alumina has a pore volume of at least 0.6 mL/g. The transition alumina preferably has a pore volume of 0.6 to 2.0 mL/g or 0.65 to 2.0 mL/g, more preferably 0.7 to 1.8 mL/g, especially 0.8 to 1.6 mL/g.

The transition alumina has a median pore diameter value of at least 15 nm. The term “median pore diameter value” is used here in order to state the median pore diameter value based on surface area, i.e. the median pore diameter (area) value is the pore diameter at the 50th percentile of the cumulative surface area curve. The transition alumina preferably has a median pore diameter value of 15 to 500 nm, more preferably 20 to 450 nm, especially 20 to 300 nm, for example 20 to 200 nm.

Mercury porosimetry and nitrogen sorption are frequently used for characterization of the pore structure for porous materials, since these methods enable the determination of porosity and of pore size distribution in one step. The two techniques are based on different physical interactions and cover particular ranges of pore size in an optimal manner.

In many cases, nitrogen sorption constitutes a sufficiently accurate method of determination, especially for relatively small pores. It is thus possible to determine the pore volume and median pore diameter value of transition aluminas by nitrogen sorption. However, larger pores may be described inadequately by nitrogen sorption.

Nitrogen sorption measurements can be conducted by means of a Micrometrics ASAP 2420. Nitrogen porosity is determined here to DIN 66134, unless stated otherwise. The analysis of pore size and of pore volume according to Barrett-Joyner-Halenda (BJH) is conducted in order to obtain the total pore volume (“cumulative pore volume from BJH desorption”) and the median pore diameter value (“average pore diameter from BJH desorption”).

Mercury porosimetry can be conducted by means of a Micrometrics AutoPore V 9600 mercury porosimeter (contact angle 140 degrees, Hg surface tension 485 dyn/cm, maximum head pressure 61000 psia). For the total pore volume and the median pore diameter value of transition aluminas, data from the pore diameter range from 3 nm to 1 µm are used.

In order to assure sufficient accuracy, the pore volume reported and the median pore diameter value of transition aluminas come from nitrogen sorption when the median pore diameter value from mercury porosimetry is less than 50 nm; or the pore volume reported and the median pore diameter value of transition aluminas come from mercury porosimetry when the median pore diameter value from mercury porosimetry is 50 nm or more.

In order to avoid distortion of the results, nitrogen sorption measurements and mercury porosimetry should be conducted on samples that have been treated in such a way that physically adsorbed species, for example moisture, have been removed from the samples. A suitable method is described hereinafter.

The BET surface area of the transition alumina can vary over a relatively wide range and can be adjusted by varying the conditions of thermal dehydration of the hydrated aluminum compounds by which the transition alumina can be obtained. The transition alumina preferably has a BET surface area in the range from 20 to 200 m²/g, more preferably 50 to 200 m²/g or 50 to 150 m²/g. BET surface area is determined to DIN ISO 9277 by means of nitrogen physisorption at 77 K, unless stated otherwise. The terms “BET surface area” and “surface area” are used here synonymously, unless stated otherwise.

Suitable transition aluminas are commercially available. In some cases, such commercial transitional aluminas are classified as “medium-porosity aluminas” or in particular as “high-porosity aluminas”. Suitable transition aluminas are, for example, products from the Puralox® TH and Puralox® TM series, both from Sasol, and products from the Versal VGL series from UOP.

The transition alumina may be used in its commercial (“unground”) form. This commercial form of alumina comprises agglomerates (secondary particles) of the individual particles or grains (primary particles). For example, a commercial alumina particle having an average (secondary) particle diameter (e.g. D₅₀) of 25 µm may comprise primary particles in sub-micrometer size. The median particle diameter (D₅₀) to which reference is made here is understood to mean the particle diameter (D₅₀) of the secondary alumina particles.

Unground transition alumina powder typically has a D₅₀ particle diameter of 10 to 100 µm, preferably 20 to 50 µm. In addition, it is possible to use transition aluminas that have been subjected to a grinding operation in order to comminute the particles to a desired size. In a suitable manner, the transition alumina can be ground in the presence of a liquid; it is preferably ground in the form of a suspension. Alternatively, the grinding can be effected by dry ball milling. Ground transition alumina powder typically has a D₅₀ particle diameter of 0.5 to 8 µm, preferably 1 to 5 µm. The particle size of transition alumina can be measured by laser diffraction particle size measuring equipment, for example a Malvern Mastersizer 2000, using water as dispersion medium. The method comprises the dispersing of the particles by an ultrasound treatment, which splits up secondary particles into primary particles. This sonication is continued until no further change in D₅₀ can be observed, for example after sonication for 3 min.

In a preferred embodiment, the transition alumina comprises at least 50% by weight, more preferably 60% to 90% by weight, of a transition alumina having an average particle size of 10 to 100 µm, especially 20 to 50 µm, based on the total weight of the transition alumina. The transition alumina may comprise a transition alumina having an average particle size of 0.5 to 8 µm, preferably 1 to 5 µm, such as not more than 50% by weight, more preferably 10% to 40% by weight, based on the total weight of the transition alumina.

The precursor material, based on the inorganic solids content, comprises not more than 30% by weight of an alumina hydrate. The precursor material, based on the inorganic solids content, preferably comprises 1% to 30% by weight of the alumina hydrate, more preferably 1% to 25% by weight, especially 1% to 20% by weight, for example 3% to 18% by weight.

The term “alumina hydrate” relates to hydrated aluminum compounds as described above, especially to aluminum hydroxides and aluminum oxyhydroxides. A discussion of the nomenclature of transition aluminas can be found in K. Wefers and C. Misra, “Oxides and Hydroxides of Aluminum”, Alcoa Laboratories, 1987. Suitable hydrated aluminum compounds comprise naturally occurring and synthetic compounds, such as aluminum trihydroxides (Al(OH)₃) such as gibbsite, bayerite and nordstrandite, or aluminum oxymonohydroxides (AIOOH) such as boehmite, pseudoboehmite and diaspore.

The alumina hydrate preferably comprises boehmite and/or pseudoboehmite. In a preferred embodiment, the total amount of boehmite and pseudoboehmite accounts for at least 80% by weight, more preferably at least 90% by weight and especially at least 95% by weight, such as 95% to 100% by weight, of the alumina hydrate. In a particularly preferred embodiment, the amount of boehmite accounts for at least 80% by weight, more preferably at least 90% by weight and especially at least 95% by weight, such as 95% to 100% by weight, of the alumina hydrate.

Suitable alumina hydrates are commercially available and comprise products from the Pural® series from Sasol, preferably products from the Pural® TH and Pural® TM series, and products from the Versal® series from UOP.

Without wishing to be bound to this assumption, it is assumed that the presence of alumina hydrate increases the mechanical stability of the support. In particular, it is assumed that nanoscale alumina hydrates of high dispersibility that are suitable for colloidal applications, for example boehmites from the Disperal® or Dispal® series from Sasol, have high binding forces and can particularly efficiently improve the mechanical stability of the support. In general, the use of such nanoscale, highly dispersible alumina hydrates for improvement of mechanical stability can enable relatively low BET surface areas under given calcination conditions.

Alumina hydrate may be partly or fully replaced by suitable alternative aluminum compounds, in which case the mechanical stability of the support is essentially maintained. Such suitable alternative aluminum compounds comprise aluminum alkoxides such as aluminum ethoxide and aluminum isopropoxide, aluminum nitrate, aluminum acetate and aluminum acetylacetonate.

The precursor material may comprise a liquid. The presence, type and amount of liquid may be chosen depending on the desired handling properties of the precursor material. For example, the presence of liquid may be desirable in order to obtain a shapable precursor material.

The liquid is typically selected from water, especially deionized water, and/or an aqueous solution comprising soluble and/or dispersible compounds selected from salts, such as ammonium acetate and ammonium carbonate; acids, such as formic acid, nitric acid, acetic acid and citric acid; bases, for example ammonia, triethylamine and methylamine; surfactants, for example triethanolamine, poloxamers, fatty acid esters and alkyl polyglycosides; particles in the sub-micrometer range, including metal oxides, for example silicon dioxide, titanium dioxide and zirconium dioxide; clays; and/or polymer particles, for example polystyrene and polyacrylates. The liquid is preferably water, especially deionized water. Typical amounts of liquid vary in the range from 10% to 60% by weight, based on the inorganic solids content of the precursor material.

The precursor material may comprise further components which may be processing aids, or which are introduced specifically for adjustment of the physical properties of the final catalyst support. The further components include pore-forming materials, lubricants, organic binders and/or inorganic binders.

The precursor material may comprise organic materials such as pore-forming materials, lubricants and organic binders in a total amount of 1.0% to 60% by weight, preferably 3% to 50% by weight, based on the total weight of the precursor material.

The precursor material may comprise lubricants and organic binders in amounts of 1.0% to 10% by weight, preferably 3% to 8% by weight, based on the total weight of the precursor material.

Pore-forming materials may be used in order to provide additional and/or broader pores in the support. The additional pore volume of broader pores can advantageously also enable more efficient impregnation of the support in the production of a catalyst. The pore-forming materials are preferably removed essentially completely in the heat treatment of the shaped bodies. Pore formation can be achieved by various mechanisms, for example by combustion (i.e. oxidation) in the presence of oxygen, decomposition, sublimation or volatilization.

Suitable pore-forming materials comprise

-   thermally decomposable materials such as oxalic acid, malonic acid,     ammonium carbonate or ammonium hydrogencarbonate; -   materials to be burnt out, for example thermally combustible     biomaterials such as acacia, sawdust and ground matter, especially     ground nutshells, such as ground pecan nut, cashew nut, walnut or     hazelnut shells; and/or -   organic polymers, for example     -   polysaccharides, such as starch, rubber, cellulose and cellulose         derivatives, including substituted celluloses such as methyl         cellulose, ethyl cellulose and carboxyethyl cellulose, and         cellulose ethers;     -   polyolefins, such as polyethylene and polypropylene;     -   aromatic hydrocarbon polymers, such as polystyrene;     -   polycarbonates, for example poly(propylene carbonate), and     -   lignins; -   carbon-containing materials such as     -   graphite;     -   pulverulent carbon-containing compounds, such as coke or         activated carbon powder, and     -   ground or unground carbon fibers.

Thermally decomposable materials such as oxalic acid, malonic acid, ammonium carbonate or ammonium hydrogencarbonate decompose on thermal treatment and break down into volatile smaller molecules that may or may not be combustible. For example, malonic acid on thermal treatment decomposes predominantly to acetic acid and carbon dioxide. Such thermally decomposable materials may offer certain advantages in industrial processes since these materials can generally be obtained from industrial sources having a degree of purity, such that they do not introduce impurities into the support.

In order to avoid the formation of an explosive atmosphere, the calcination of the shaped bodies using thermally decomposable materials is preferably conducted in an atmosphere with reduced oxygen content, for example not more than 10% by volume or not more than 5% by volume. When the thermal decomposition is effected at relatively low temperatures, the process can be reliably controlled well below the ignition temperature of potentially combustible molecules formed in the decomposition of the decomposable materials. This can enable safe performance of the thermal treatment even in the case of relatively high concentrations of oxygen in the atmosphere within the apparatus for the thermal treatment. In this case, it is possible to use an air atmosphere.

Suitable lubricants comprise

-   graphite; -   vaseline, mineral oil or lubricating grease; -   fatty acids, such as stearic acid or palmitic acid; salts of fatty     acids, for example stearates such as potassium stearate, magnesium     stearate and aluminum stearate, or palmitates such as potassium     palmitate, magnesium palmitate and aluminum palmitate; fatty acid     derivatives, such as esters of fatty acids, especially esters of     saturated fatty acids, such as stearate esters such as methyl and     ethyl stearate; and/or -   shapable organic solids, for example waxes such as paraffin wax and     cetyl palmitate.

It is preferable that the lubricant does not introduce any inorganic impurities into the catalyst support. Among the abovementioned lubricants, preference is given to graphite, stearic acid, aluminum stearate and combinations thereof.

Organic binders, which are sometimes also referred to as “temporary binders”, may be used in order to improve the shapability of the precursor material and in order to maintain the integrity of the “green” phase, i.e. the unbaked phase in which the mixture is shaped to shaped bodies. The organic binders are preferably removed essentially completely during the heat treatment of the shaped bodies.

Suitable organic binders comprise

-   polyvinyllactam polymers, such as polyvinylpyrrolidones, or     vinylpyrrolidone copolymers, such as vinylpyrrolidone-vinyl acetate     copolymers; -   alcohols, especially polyols such as glycol or glycerol; and/or -   polyalkylene glycols, for example polyethylene glycol.

Advantageously, the pore-forming materials and processing aids, for example organic binders and lubricants, have a low ash content. The term “ash content” is understood to mean the noncombustible fraction that remains after the combustion of the organic materials under air at high temperature, i.e. after the heat treatment of the shaped bodies. The ash content is preferably below 0.1% by weight, based on the total weight of the organic materials.

In addition, pore-forming materials and processing aids, for example organic binders and lubricants, in the course of heat treatment of the shaped bodies, i.e. in the course of thermal decomposition or combustion, preferably do not form any significant amounts of further volatile combustible constituents, for example carbon monoxide, ammonia or combustible organic compounds. An excess of volatile organic constituents may result in an explosive atmosphere. For the process step of combustion or decomposition, preference is given to employing an appropriate safety concept.

Inorganic binders are permanent binders that contribute to sufficient bonding of the alumina particles and increase the mechanical stability of the shaped alpha-alumina bodies. The inorganic binders include those that form exclusively alumina on calcination. For the purposes of this application, these inorganic binders are referred to as intrinsic inorganic binders. Such intrinsic inorganic binders include alumina hydrates, as described above.

Extrinsic inorganic binders, by contrast, do not form exclusively alumina on calcination. Suitable extrinsic inorganic binders are all inorganic species that are customarily used in the specialist field, for example silicon-containing species such as silicon dioxide or silicates, including clays such as kaolinite, or metal hydroxides, metal carbonates, metal nitrates, metal acetates, or metal oxides such as zirconium dioxide, titanium oxide or alkaline metal oxides. Since extrinsic inorganic binders introduce impurities that can have an adverse effect on catalyst performance, they are preferably present in controlled amounts. The precursor material preferably comprises extrinsic inorganic binders in amounts of 0.0% to 5.0% by weight, preferably 0.05% to 1.0% by weight, based on the inorganic solids content of the precursor material. In a preferred embodiment, the precursor material does not comprise any extrinsic inorganic binder.

The precursor material is typically provided by dry mixing of the components thereof and optionally subsequent addition of the liquid. The precursor material may be shaped to shaped bodies by extrusion, tableting, pelletization, casting, shaping or microextrusion, especially by extrusion or tableting.

The size and shape of the shaped bodies and hence of the catalyst is chosen such that suitable packing of the catalysts obtained from the shaped bodies in a reactor tube is possible. The catalysts obtained from the shaped bodies suitable for the catalysts of the invention are preferably used in reactor tubes having a length of 6 to 14 m and an internal diameter of 20 mm to 50 mm. In general, the support consists of individual bodies having a maximum extent in the range from 3 to 20 mm, such as 4 to 15 mm, especially 5 to 12 mm. The maximum extent is understood to mean the longest straight line between two points on the outer circumference of the support.

The shape of the shaped bodies is not particularly restricted and may have any technically possible shape, for example depending on the shaping process. The support may, for example, be a solid extrudate or a hollow extrudate, for example a hollow cylinder. In another embodiment, the support may be characterized by a multilobal structure. A multilobal structure is understood to mean a cylinder structure having a multitude of cavities, for example grooves or chamfers, which run over the circumference of the cylinder along the height of the cylinder. In general, the cavities are in an essentially equidistant arrangement around the circumference of the cylinder.

The support preferably has the shape of a solid extrudate, for example pellets or cylinders, or of a hollow extrudate, for example a hollow cylinder. In a preferred embodiment, the shaped bodies are shaped by extrusion, for example microextrusion. In this case, the precursor material appropriately comprises a liquid, especially water, in order to form a shapable precursor material.

In a preferred embodiment, the extrusion comprises the introducing of at least one solid component into a mixing apparatus before the liquid is added. Preference is given to mixing using a roller-based mixer (Mix-Muller, H roller) or a horizontal mixer, for example a Ploughshare® mixer (from Gebrüder Lödige Maschinenbau). The shaping of an extrudable paste to give the precursor material can be monitored and controlled with reference to data that reflect the performance required from the mixing apparatus.

The precursor material is typically extruded through a shaping orifice. The cross section of the shaping orifice is chosen in accordance with the desired geometry of the shaped body.

The shaping orifice used for the extrusion may comprise a die and mandrels, in which case the die essentially determines the outer surface area of the shaped bodies and the mandrels essentially determine the shape, size and position of any passages. Suitable extrusion tools are described, for example, in WO2019/219892 A1.

The geometry of the shape of the shaped bodies is defined by the geometry of the extrusion apparatus through which the precursor material is extruded. In general, the geometry of the shape of the extrudate differs slightly from the geometry of the extrusion apparatus, but has essentially the geometric properties described above. The absolute dimensions of the shape are generally somewhat lower than the dimensions of the extrudate, which is attributable to the high temperatures required for formation of alpha-alumina, and to the shrinkage on cooling of the extrudate. The extent of shrinkage depends on the temperatures employed in the calcining and the constituents of the shaped bodies. Therefore, the size of the shaping orifice used for the extrusion should routinely be adjusted so as to take account of the extrudate shrinkage during the subsequent calcining.

If the shaped body comprises multiple passages, the axes of the passages typically run parallel. However, the shaped bodies may be slightly bent or twisted about their z axis (height). The shape of the cross section of the passages may differ slightly from the desired ideal geometric forms described above. In the case of a large number of shaped bodies, individual passages of a small number of shaped catalyst bodies may be closed. Typically, the end faces of the shaped catalyst bodies in the xy plane, as a result of production, are more or less uneven and not smooth. The height of the shaped bodies (length of the shaped bodies in z direction) is generally not exactly the same for all shaped bodies, but forms a distribution with an average height as arithmetic average.

The extrudate is preferably cut to the desired length while still wet. The extrudate is preferably cut at an angle essentially perpendicular to its circumferential face. In order to reduce unwanted variances from the geometry of the extrusion apparatus, the extrudate may alternatively be cut at an oblique angle of up to 30°, e.g. 10° or 20°, relative to the angle perpendicular to the circumferential face of the extrudate.

Variances from the geometry as occur in the extrusion process and/or in the further processing of the extrudate, for example a cutting step, may in principle also exist in the porous catalyst support of the invention without significantly reducing the advantageous effects of its pore structure. It will be clear to the person skilled in the art that perfect geometric forms fundamentally cannot be achieved owing to the inaccuracy inherent to a certain degree in all production processes.

In another embodiment, the precursor material is shaped to shaped bodies with the aid of a microextrusion process as described in WO 2019/072597 A1.

In another embodiment, the precursor material is shaped to shaped bodies by a gel casting process as described in WO 2020/053563 A1.

In a further embodiment, the precursor material is shaped to shaped bodies by tableting. In this case, the precursor material typically does not comprise any liquid. Tableting is a method of pressure agglomeration. A pulverulent or previously agglomerated bulk material is introduced into a press mold having a die between two punches and compacted by monoaxial compression and shaped to a solid compact. This operation is divided into four parts: metered introduction, compaction (elastic deformation), plastic deformation and ejection. Tableting is conducted, for example, on rotary presses or eccentric presses.

If desired, the upper punch and/or the lower punch may have protruding pins in order to form internal passages. It is also possible to provide the press punches with a multitude of movable pins, such that a punch can be provided, for example, with four pins in order to produce shaped bodies having four holes (passages). Typical configurations of such tools can be found, for example, in US 8,865,614 B2.

The compression force in tableting influences the compaction of the bulk material. In practice, it has been found to be appropriate to control the side crushing strength of the porous catalyst support through choice of the corresponding compression force and to check it by random sampling. In the context of the present invention, side crushing strength is the force that breaks the porous catalyst support present between two flat parallel plates, with the two flat parallel end faces of the catalyst support at right angles to the flat parallel plates.

For tableting, it is often advantageous to use lubricants, especially those mentioned above. For improvement of tableting properties, it is possible to use a pre-pelletization and/or sieving step. For the pre-pelletization, it is possible to use a roll compressor, for example a Chilsonator® from Fitzpatrick. Further information on tableting, especially on pre-pelletization, sieving, lubricants and tools, can be found in WO2010/000720 A2.

Prior to the calcining, the shaped bodies may be dried, especially when the precursor material comprises a liquid. The drying is appropriately effected at temperatures in the range from 20 to 400° C., especially 30 to 300° C., for example 70 to 150° C. The drying is typically effected over a period of up to 100 h, preferably 0.5 h to 30 h, more preferably 1 h to 16 h.

The drying can be effected in any atmosphere, for example in an oxygenous atmosphere such as air, in nitrogen or in helium or in mixtures thereof, preferably in air. The drying is typically conducted in an oven. The type of furnace is not particularly restricted. It is possible, for example, to use stationary air circulation ovens, rotatable cylinder ovens or tunnel ovens. The heat can be supplied directly and/or indirectly.

Preference is given to using flue gas (offgas) from a combustion process at a suitable temperature for the drying step. The flue gas may be used in diluted or undiluted form in order to enable direct heating and to remove evaporated moisture and other components released from the shaped bodies. The offgas is typically passed through an oven as described above. In another preferred embodiment, the offgas from a calcination process step is used for direct heating.

Drying and calcination may be conducted successively in separate equipment, and in a batchwise or continuous process. It is possible to employ intermediate cooling. In another embodiment, drying and calcination are conducted in the same equipment. In a batchwise process, it is possible to employ a time-resolved temperature ramp (program). In a continuous process, it is possible to employ a spatially resolved temperature ramp (program), for example when the shaped bodies are conducted continuously through regions (zones) at different temperatures.

Preference is given to improving energy efficiency using the thermal integration measures known from the prior art. For example, it is thus possible to use relatively hotter offgases from a process step or a process stage for heating of the feed gas, the equipment or the shaped bodies in a different process step or a different process stage by direct (admixing) or indirect (heat exchanger) means. It is likewise also possible to utilize heat integration for cooling of relatively hotter offgas streams prior to further treatment or execution.

The shaped bodies are calcined in order to obtain the porous catalyst support. The calcination temperature and time are thus sufficient to convert at least a portion of the transition alumina to alpha-alumina, which means that at least a portion of the metastable alumina phases in the transition alumina is converted to alpha-alumina.

Typically, the calcination is effected at a temperature of at least 1100° C., such as at least 1300° C., preferably at least 1350° C., more preferably at least 1400° C., especially at least 1450° C. The calcination is preferably effected at an absolute pressure in the range from 0.5 bar to 35 bar, especially in the range from 0.9 to 1.1 bar, for example at atmospheric pressure (about 1013 mbar). The total duration of the heating is typically in the range from 0.5 to 100 h, preferably from 2 to 20 h.

The calcination is typically conducted in a furnace. The type of furnace is not particularly restricted. It is possible, for example, to use ovens such as stationary air circulation furnaces, rotary furnaces or tunnel furnaces, or combustion furnaces such as rotary combustion furnaces or tunnel combustion furnaces, especially roller hearth furnaces.

The calcination can be conducted in any atmosphere, for example in an oxygenous atmosphere such as air, in nitrogen or in helium, or in mixtures thereof. Preferably, especially when the shaped bodies comprise a thermally decomposable material or a material to be burnt out, the calcination is conducted at least partly or completely in an oxidizing atmosphere, such as in an oxygenous atmosphere such as air.

As described above, pore-forming materials and processing auxiliaries, for example organic binders and lubricants, preferably do not form any significant amounts of further volatile combustible components, for example carbon monoxide or combustible organic compounds, on calcination of the shaped bodies. An explosive atmosphere can also be avoided by limiting the oxygen concentration in the atmosphere during the calcination, for example to oxygen concentration below the limiting oxygen concentration (LOC) with respect to the further combustible components. The LOC, also called minimum oxygen concentration (MOC), is the limiting concentration of oxygen below which combustion is impossible.

It is appropriately possible to use lean air or a gaseous recycle stream with a limited oxygen content together with a stream for oxygen enrichment that also compensates for gaseous purge streams. In an alternative approach, it is possible to avoid an explosive atmosphere by limiting the rate of formation of further combustible components. The rate of formation of further combustible components can be limited by gradual heating to the calcination temperature or by stepwise heating. In the case of stepwise heating, the temperature is appropriately kept for several hours at the approximate combustion temperature and then heated up to temperatures of 1000° C. In a continuous calcination process, it is also possible to control the feed rate of the shaped bodies to the calcination apparatus, for example the furnace, such that the rate of formation of further combustible components is limited.

According to the type of pore-forming materials, lubricants, organic binders and gaseous constituents, it is possible to employ an offgas treatment in order to clean the offgases obtained in the calcination. It is preferably possible to use an acidic or alkaline absorber, a flare or catalytic combustion, a deNOx treatment or combinations thereof for offgas treatment.

Preference is given to stepwise heating. In the case of stepwise heating, the shaped bodies may be positioned in a highly clean and inert, refractory combustion capsule which is moved through an oven having several heating zones, e.g. 2 to 8 or 2 to 5 heating zones. The inner refractory fuel capsule may consist of alpha-alumina or corundum, especially alpha-alumina.

The porous catalyst support typically has a BET surface area in the range from 0.5 to 5.0 m²/g. The porous catalyst support preferably has a BET surface area in the range from 1.0 to 4.0 m²/g, more preferably 1.5 to 3.0 m²/g, especially 1.7 to 2.5 m²/g, such as 1.8 to 2.2 m²/g.

The porous catalyst support typically has a pore volume present in pores having a diameter in the range from 0.1 to 1 µm of at least 40% of the total pore volume as determined by mercury porosimetry. The porous catalyst support preferably has a pore volume present in pores having a diameter in the range from 0.1 to 1 µm of at least 50% of the total pore volume, more preferably at least 55% of the total pore volume, especially at least 60% of the total pore volume, such as at least 65% or at least 70% of the total pore volume. The porous catalyst support typically has a pore volume present in pores having a diameter in the range from 0.1 to 1 µm of preferably 40% to 99%, more preferably 45% to 99%, especially 50% to 97%, of the total pore volume.

The porous catalyst support typically has a ratio r_(pv) of the pore volume present in pores having a diameter in the range from more than 1 to 10 µm to the pore volume present in pores having a diameter in the range from 0.1 to 1 µm of not more than 0.50. The ratio r_(pv) is preferably in the range from 0.0 to 0.45, more preferably 0.0 to 0.40 or 0.0 to 0.35.

The porous support comprises at least 85% by weight, preferably at least 90% by weight, more preferably at least 95% by weight, especially at least 97.5% by weight, of alpha-alumina, based on the total weight of the support. The amount of alpha-alumina can be determined, for example, by x-ray diffraction analysis.

In one embodiment, the porous shaped catalyst support body has

-   a BET surface area in the range from 0.5 to 5.0 m²/g; -   a total pore volume of at least 0.5 mL/g, determined by mercury     porosimetry; and -   a pore volume present in pores having a diameter in the range from     0.1 to 1 µm of at least 40% of the total pore volume as determined     by mercury porosimetry; wherein the ratio r_(pv) of the pore volume     present in pores having a diameter in the range from more than 1 to     10 µm to the pore volume present in pores having a diameter in the     range from 0.1 to 1 µm is not more than 0.50.

In a preferred embodiment, the porous support is in the form of individual shaped bodies, for example in a form as described above. The porous catalyst support preferably takes the form of individual shaped bodies having an outer surface, a first lateral face, a second lateral face and at least one inner passage that extends from the first lateral face to the second lateral face.

The ratio of the geometric surface area of the catalyst support SA_(geo) to the geometric volume of the catalyst support V_(geo) (SA_(geo)/V_(geo)) is preferably at least 1.1 mm⁻¹ and at most 10 mm⁻¹. Preferably, the ratio of SA_(geo) to V_(geo) is in the range from 1.15 mm⁻¹ to 5.0 mm⁻¹, more preferably in the range from 1.2 mm⁻¹ to 2.0 mm⁻¹. The geometric surface area SA_(geo) and the geometric volume V_(geo) are found from the external, macroscopic dimensions of the porous catalyst support, taking account of the cross-sectional area, the height and optionally the number of internal passages. In other words, the geometric volume V_(geo) of the catalyst support is the volume of a solid body having the same outer dimensions minus the volume occupied by the passages. The geometric surface area SA_(geo) is likewise composed of the circumferential area, the first and second end surfaces, and optionally the surface area that defines the passages. The first/second end surface is the face enclosed by the circumferential line of the end face, minus the cross-sectional areas of the passages. The surface area that defines the passages is the face that lines the passages.

A ratio of SA_(geo) to V_(geo) within the preferred range enables better contact of the reaction gases with the catalyst surface, which promotes the conversion of the reactants and limits internal diffusion phenomena, which leads to an increase in reaction selectivity.

The porous shaped catalyst support body preferably does not have any washcoat particles or a washcoat layer on its surface in order to fully maintain the porosity of the uncoated support.

The porous shaped catalyst support body may comprise impurities, for example sodium, potassium, magnesium, calcium, silicon, iron, titanium and/or zirconium. Such impurities can be introduced by constituents of the precursor material, especially inorganic binders or auxiliaries for improvement of mechanical stability. In one embodiment, the porous shaped catalyst support body comprises

-   a total amount of up to 1500 ppmw of sodium and potassium; -   up to 2000 ppmw of calcium; -   up to 1000 ppmw of magnesium; -   up to 10000 ppmw of silicon; -   up to 1000 ppmw of titanium; -   up to 1000 ppmw of iron; and/or -   up to 10000 ppmw of zirconium;

based on the total weight of the support.

A low sodium content is preferred in order to prevent secretion of the supported metal and to avoid any change in the supported component.

The invention also relates to a shaped catalyst body for preparation of ethylene oxide by selective gas phase oxidation (epoxidation) of ethylene, i.e. an epoxidation catalyst, comprising at least 15% by weight of silver, based on the total weight of the shaped catalyst body, deposited on a porous catalyst support as described above.

The shaped catalyst body typically comprises 15% to 70% by weight of silver, preferably 20% to 60% by weight of silver, more preferably 25% to 50% by weight or 30% to 50% by weight of silver, based on the total weight of the shaped catalyst body. A silver content within this range enables an advantageous equilibrium between the conversion induced by each shaped catalyst body and the cost efficiency of production of the shaped catalyst body.

As well as silver, the shaped catalyst body may comprise one or more promoter species. A promoter species refers to a component that results in an improvement in one or more of the catalytic properties of the catalyst compared to a catalyst that does not comprise that component. The promoter species may be any of the species known in the specialist field that improve the catalytic properties of the silver catalyst. Examples of catalytic properties are operation capability (runaway resistance), selectivity, activity, conversion and longevity of the catalyst.

The shaped catalyst body may comprise a transition metal or a mixture of two or more transition metals in an amount effective as a promoter. Suitable transition metals may, for example, be the elements from groups IIIB (scandium group), IVB (titanium group), VB (vanadium group), VIB (chromium group), VIIB (manganese group), VIIIB (iron, cobalt, nickel groups), IB (copper group) and IIB (zinc group) of the Periodic Table of the Elements, and combinations thereof. Typically, the transition metal is an early transition metal, i.e. from groups IIIB, IVB, VB or VIB, for example hafnium, yttrium, molybdenum, tungsten, rhenium, chromium, titanium, zirconium, vanadium, tantalum, niobium or a combination thereof. In one embodiment, the transition metal promoter(s) is/are present in a total amount of 150 ppm to 5000 ppm, typically 225 ppm to 4000 ppm, most typically from 300 ppm to 3000 ppm, reported as metal(s) relative to the total weight of the shaped catalyst body.

Among the transition metal promoters listed, rhenium (Re) is a particularly effective promoter for ethylene epoxidation catalysts with high selectivity. The rhenium component in the shaped catalyst body may be in any suitable form, but is typically one or more rhenium-containing compounds (e.g. a rhenium oxide) or complexes.

In some embodiments, the shaped catalyst body may comprise an alkali metal or a mixture of two or more alkali metals in an amount effective as a promoter. Suitable alkali metal promoters are, for example, lithium, sodium, potassium, rubidium, cesium or combinations thereof. The amount of alkali metal, e.g. potassium, is typically in the range from 50 ppm to 5000 ppm, preferably from 300 ppm to 2500 ppm, more preferably from 500 ppm to 1500 ppm, based on the alkali metal relative to the total weight of the shaped catalyst body. The amount of alkali metal is determined by the amount of alkali metal which is introduced by the porous catalyst support and the amount of alkali metal which is introduced by the impregnation solution described below.

Particular preference is given to combinations of heavy alkali metals such as cesium (Cs) or rubidium (Rb) with light alkali metals such as lithium (Li), sodium (Na) and potassium (K).

The shaped catalyst body may also comprise an alkaline earth metal of group IIA or a mixture of two or more alkaline earth metals of group IIA. Suitable alkaline earth metal promoters are, for example, beryllium, magnesium, calcium, strontium and barium or combinations thereof. The amounts of alkaline earth metal promoters may be used in similar amounts as for the alkali metal or transition metal promoters.

The shaped catalyst body may also comprise a main group element or a mixture of two or more main group elements in an amount effective as a promoter. Suitable main group elements comprise any of the elements in groups IIIA (boron group) to VIIA (halogen group) of the Periodic Table of the Elements. For example, the shaped catalyst body may comprise sulfur, phosphorus, boron, halogen (e.g. fluorine), gallium or a combination thereof in an amount effective as a promoter.

The shaped catalyst body may also comprise a rare earth metal or a mixture of two or more rare earth metals in an amount effective as a promoter. The rare earth metals comprises any of the elements having an atomic number of 57 to 103. Some examples of these elements are lanthanum (La), cerium (Ce) and samarium (Sm). The rare earth metal promoters may be used in similar amounts as for the transition metal promoters.

The invention also relates to a process for producing a shaped catalyst body as described above, in which

-   a) a porous shaped catalyst support body as described above is     impregnated with a silver impregnation solution, preferably under     reduced pressure; and the impregnated porous shaped catalyst support     body is optionally subjected to drying; and -   b) the impregnated porous shaped catalyst support body is subjected     to a heat treatment;

wherein steps a) and b) are optionally repeated.

It will be apparent that all embodiments of the shaped catalyst body are also applicable, if appropriate, to the process for producing the shaped catalyst body.

In order to obtain a shaped catalyst body having high silver contents, steps i) and ii) may be repeated multiple times. In this case, it is assumed that the catalyst intermediate obtained after the first (or subsequent up to penultimate) impregnation/calcination cycle comprises a portion of the total amount of target Ag and/or promoter concentrations. The catalyst intermediate is then again impregnated with the silver impregnation solution and calcined in order to obtain the target Ag and/or promoter concentrations.

It is possible to use any silver impregnation solution known in the specialist field that is suitable for impregnation of a refractory support. Silver impregnation solutions typically comprise a silver carboxylate, for example silver oxalate, or a combination of a silver carboxylate and oxalic acid, in the presence of an aminic complexing agent, for example a C₁-C₁₀-alkylenediamine, especially ethylenediamine. Suitable impregnation solutions are described in EP 0 716 884 A2, EP 1 115 486 A1, EP 1 613 428 A1, US 4,731,350 A, WO 2004/094055 A2, WO 2009/029419 A1, WO 2015/095508 A1, US 4,356,312 A, US 5,187,140 A, US 4,908,343 A, US 5,504,053 A and WO 2014/105770 A1. A discussion of suitable silver impregnation solutions can also be found in Kunz, C. et al., On the Nature of Crystals Precipitate from Aqueous Silver Ethylenediamine Oxalate Complex Solutions, Z. Anorg. Allg. Chem., 2021, 647, DOI: 10.1002/zaac.202100079.

During the heat treatment, liquid constituents of the silver impregnation solution evaporate, which results in precipitation of a silver compound comprising silver ions out of the solution and deposition on the porous support. At least some of the silver ions deposited are then converted to metallic silver on further heating. Preferably at least 70 mol% of the silver compounds, more preferably at least 90 mol%, even more preferably at least 95 mol% and especially at least 99.5 mol% or at least 99.9 mol%, i.e. essentially all silver ions, based on the total molar amount of silver in the impregnated porous catalyst support. The amount of silver ions converted to metallic silver can be determined, for example, by means of x-ray diffraction patterns (XRD).

The heat treatment may also be referred to as calcination process. It is possible to use any of the calcination processes known for the purpose in the specialist field. Suitable examples of calcination methods are described in US 5,504,052 A, US 5,646,087 A, US 7,553,795 A, US 8,378,129 A, US 8,546,297 A, US 2014/0187417 A1, EP 1 893 331 A1 or WO 2012/140614 A1. The heat treatment can be conducted in a continuous method or with at least partial recycling of the calcination gas.

The heat treatment is typically conducted in an oven. The type of furnace is not particularly restricted. It is possible, for example, to use stationary air circulation ovens, rotary ovens or tunnel ovens. In one embodiment, the heat treatment involves passing a heated gas stream over the impregnated bodies. The duration of heat treatment is generally in the range from 5 min to 20 h, preferably 5 min to 30 min.

The heat treatment temperature is generally in the range from 200 to 800° C., preferably 210 to 650° C., more preferably 220 to 500° C., especially 220 to 350° C. The heating rate within the temperature range from 40 to 200° C. is preferably at least 20 K/min, more preferably at least 25 K/min, for example at least 30 K/min. A high heating rate can be achieved by passing a heated gas at a high gas flow rate over the impregnated refractory support or the impregnated catalyst intermediate.

A suitable throughput for the gas may be in the range from, for example, 1 to 1000 m³/h (STP), 10 to 1000 m³/h (STP), 15 to 500 m³/h (STP) or 20 to 300 m³/h (STP) per kg of impregnated bodies. The term “kg of impregnated bodies” in a continuous process is understood to mean the amount of impregnated bodies (in kg/h) multiplied by the time (in hours) for which the gas stream is passed over the impregnated bodies. It has been found that, when the gas stream is passed over relatively large amounts of impregnated bodies, for example 15 to 150 kg of impregnated bodies, the flow rate may be chosen in the lower part of the ranges described above, in which case the desired effect is achieved.

The direct determination of the temperature of the heated impregnated bodies may present difficulties in practice. Therefore, when a heated gas is passed over the impregnated bodies during the heat treatment, the temperature of the heated impregnated bodies is considered to be the temperature of the gas immediately after the gas has passed over the impregnated bodies. In a practical embodiment, the impregnated bodies are positioned on a suitable surface, for example a wire braid or a perforated calcination belt, and the temperature of the gas is measured by one or more thermocouples disposed alongside the opposite side of the impregnated bodies that comes into contact with the gas first. The thermocouples are appropriately arranged close to the impregnated bodies, for example at a distance of 1 to 30 mm, for example 1 to 3 mm or 15 to 20 mm, from the impregnated bodies.

The use of multiple thermocouples can improve the accuracy of temperature measurement. If multiple thermocouples are used, these may be distributed uniformly over the surface on which the impregnated bodies lie on the wire mesh, or over the width of the perforated calcination belt. The average is considered to be the temperature of the gas immediately after the gas has passed over the impregnated bodies. In order to heat the impregnated bodies to the temperatures described above, the gas typically has a temperature of 220 to 800° C., preferably 230 to 550° C., especially 240 to 350° C.

Preference is given to stepwise heating. In the case of stepwise heating, the impregnated bodies are positioned on a conveyor belt which moves through an oven having several heating zones, e.g. 2 to 8 or 2 to 5 heating zones. The heat treatment is preferably effected in an inert atmosphere, e.g. nitrogen, helium or mixtures thereof, especially in nitrogen.

The invention further relates to a process for preparing ethylene oxide by selective gas phase oxidation (epoxidation) of ethylene, comprising the reaction of ethylene and oxygen in the presence of a shaped catalyst body as described above.

It will be apparent that all embodiments of the shaped catalyst body are also applicable, if appropriate, to the process for preparing ethylene oxide in the presence of the shaped catalyst body.

The epoxidation can be performed by any of the methods known to the person skilled in the art. It is possible to use any of the reactors that may be used in the prior art ethylene oxide preparation processes, for example externally cooled shell and tube reactors (cf. Ullmann’s Encyclopedia of Industrial Chemistry, 5th edition, vol. A-10, p. 117-135, 123-125, VCH-Verlagsgesellschaft, Weinheim 1987) or reactors with a loose catalyst bed and cooling tubes, for example the reactors described in DE 34 14 717 A1, EP 0 082 609 A1 and EP 0 339 748 A2.

The epoxidation is preferably conducted in at least one tubular reactor, preferably in a shell and tube reactor. On an industrial scale, ethylene epoxidation is preferably conducted in a multitube reactor comprising several thousand tubes. The catalyst is introduced into the tubes that are present within a shell filled with a coolant. In commercial applications, the internal tube diameter is typically in the range from 20 to 40 mm (see, for example, US 4,921,681 A) or more than 40 mm (see, for example, WO 2006/102189 A1).

For preparation of ethylene oxide from ethylene and oxygen, the reaction can be conducted under customary reaction conditions as described, for example, in DE 25 21 906 A, EP 0 014 457 A2, DE 23 00 512 A1, EP 0 172 565 A2, DE 24 54 972 A1, EP 0 357 293 A1, EP 0 266 015 A1, EP 0 085 237 A1, EP 0 082 609 A1 and EP 0 339 748 A2. It is additionally possible to add inert gases such as nitrogen, or gases that are inert under the reaction conditions, for example steam, methane, and optionally reaction moderators, for example halohydrocarbons such as ethyl chloride, vinyl chloride or 1,2-dichloroethane, to the reaction gas composed of ethylene and molecular oxygen.

The oxygen content of the reaction gas is preferably within a range in which there are no explosive gas mixtures. A suitable composition of the reaction gas for preparation of ethylene oxide may comprise, for example, an amount of ethylene in the range from 10% to 80% by volume, preferably from 20% to 60% by volume. The oxygen content of the reaction gas is preferably in the region of not more than 10% by volume, preferably not more than 9% by volume, more preferably not more than 8% by volume and most preferably not more than 7.5% by volume.

The reaction gas preferably comprises a chlorine-containing reaction moderator such as ethyl chloride, vinyl chloride or 1,2-dichloroethane in an amount of 0 to 15 ppm by weight, preferably in an amount of 0.1 to 8 ppm by weight, based on the total weight of the reaction gas. The rest of the reaction gas generally consists of hydrocarbons such as methane and also inert gases such as nitrogen. Furthermore, it is also possible for other substances such as steam, carbon dioxide or noble gases to be present in the reaction gas.

The concentration of the carbon dioxide in the feed (i.e. the gas mixture which is supplied to the reactor) typically depends on the selectivity of the catalyst and the efficiency of the apparatuses for removal of the carbon dioxide. The carbon dioxide concentration in the feed is preferably not more than 3% by volume, more preferably less than 2% by volume, especially less than 1% by volume. An illustrative plant for removal of carbon dioxide is described in US 6,452,027 B1.

The above-described constituents of the reaction mixture may each optionally include small amounts of impurities. Ethylene may be used, for example, in any purity suitable for the gas phase oxidation of the invention. Suitable purities comprise, but are not limited to, polymer grade ethylene, typically having a purity of at least 99%, and chemical grade ethylene, typically having a purity of less than 95%. The impurities typically consist especially of ethane, propane and/or propene.

The reaction or oxidation of ethylene to ethylene oxide is typically conducted at elevated catalyst temperatures. Preference is given to catalyst temperatures in the range from 150 to 350° C., more preferably 180 to 300° C., even more preferably 190 to 280° C. and especially 200 to 280° C. The present invention therefore also provides a process as described above, in which the oxidation is conducted at a catalyst temperature in the range from 180 to 300° C., preferably 200 to 280° C. The catalyst temperature may be determined by thermocouples within the catalyst bed. As used here, the catalyst temperature or the temperature of the catalyst bed is considered to be the weight-average temperature of the catalyst particles.

The reaction (oxidation) of the invention is preferably conducted at pressures in the range from 5 to 30 bar. All pressures here are absolute pressures, unless stated otherwise. Particular preference is given to performing the oxidation at a pressure in the range from 5 to 25 bar, for example 10 bar to 24 bar and especially 14 bar to 23 bar. The present invention therefore also provides a process as described above, in which the oxidation is conducted at a pressure in the range from 14 bar to 23 bar.

It has been found that the physical properties of the shaped catalyst body, especially the BET surface area and pore size distribution, have a significant positive effect on catalyst selectivity. This effect is particularly marked when the catalyst is operated at very high productivities (work rates), i.e. at high olefin oxide production.

The process of the invention is preferably conducted under conditions suitable for obtaining a reaction mixture having an ethylene oxide content of at least 2.3% by volume. In other words, the ethylene oxide discharge concentration (ethylene oxide concentration at the reactor outlet) is preferably at least 2.3% by volume. The ethylene oxide discharge concentration is preferably in the range from 2.5% to 4.0% by volume, more preferably in the range from 2.7% to 3.5% by volume.

The oxidation is preferably conducted in a continuous process. If the reaction is conducted continuously, the GHSV (gas hourly space velocity), according to the reactor type chosen, for example size/cross-sectional area of the reactor, shape and size of the catalyst, is preferably in the range from 800 to 10000/h, more preferably in the range from 2000 to 8000/h, even more preferably in the range from 2500 to 6000/h, especially in the range from 4500 to 5500/h, where the values reported are based on the catalyst volume.

In a further embodiment, the present invention is also directed to a process as disclosed above for preparation of ethylene oxide (EO) by gas phase oxidation of ethylene by means of oxygen, wherein the measured EO space-time yield is greater than 180 kg_(EO)/(m³ _(cat)h), preferably to an EO space-time yield of greater than 200 kg_(EO)/(m³ _(cat)h), such as greater than 250 kg_(EO)/(m³ _(cat)h), greater than 280 kg_(EO)/(m³ _(cat)h) or greater than 300 kg_(EO)/(m³ _(cat)h). Preferably, the measured EO space-time yield is less than 500 kg_(EO)/(m³ _(cat)h); more preferably, the EO space-time yield is less than 350 kg_(EO)/(m³ _(cat)h).

The preparation of ethylene oxide from ethylene and oxygen can advantageously be conducted in a circulation process. After each pass, the newly formed ethylene oxide and the by-products formed in the reaction are removed from the product gas stream. The remaining gas stream is supplemented with the requisite amounts of ethylene, oxygen and reaction moderators and introduced back into the reactor. The ethylene oxide can be separated from the product gas stream and worked up by the customary prior art methods (cf. Ullmanns Enzyklopädie der industriellen Chemie [Ullmann’s Encyclopedia of Industrial Chemistry], 5th edition, vol. A-10, p. 117-135, 123-125, VCH-Verlagsgesellschaft, Weinheim 1987).

The invention is illustrated in detail by the appended drawings and the examples that follow.

FIG. 1 is an illustrative surface-rendered three-dimensional FIB-SEM tomogram of a 10 µm x 10 µm x 10 µm cubic volume of a porous metal-on-Al₂O₃ catalyst obtained after segmentation. The solid material marked as (i) corresponds to the Al₂O₃ backbone that bounds the pores that appear marked as (ii).

FIG. 2 is an illustrative surface-rendered three-dimensional FIB-SEM tomogram of a 10 µm x 10 µm x 10 µm cubic volume of a porous metal-on-Al₂O₃ catalyst obtained after segmentation. The figure shows solely voxels corresponding to the supported metal.

FIG. 3 is an illustrative graphic diagram of a pore network model that has been calculated for a FIB-SEM tomogram of a 10 µm x 10 µm x 10 µm cubic volume of a porous supported metal-on-Al₂O₃ catalyst. The figure shows pore regions as spheres having different volume, connected by cylindrical necks.

FIG. 4 shows the shape of the porous alpha-alumina catalyst supports A and B.

FIG. 5 shows logarithmic differential intrusion [mL/g] and cumulative intrusion [mL/g] relative to the pore size (pore diameter) [µm] of the inventive porous catalyst support A.

FIG. 6 shows logarithmic differential intrusion [mL/g] and cumulative intrusion [mL/g] relative to the pore size (pore diameter) [µm] of porous catalyst support B (comparative example).

FIG. 7 shows a representative scanning electron micrograph of a cross section that has been exposed by machining of the comparative catalyst 2 by means of a focused ion beam. The pores appear black, the Al₂O₃ backbone appears light gray, and the supported silver particles appear as white bodies. The scale bar has a horizontal length of 2000 nanometers.

FIG. 8 shows a representative scanning electron micrograph of a cross section that has been exposed by machining of the inventive catalyst 1 by means of a focused ion beam. The pores appear black, the Al2O3 backbone appears light gray, and supported silver particles appear as white objects. The scale bar has a horizontal length of 2000 nanometers.

METHOD 1: NITROGEN SORPTION

Nitrogen sorption measurements were conducted by means of a Micrometrics ASAP 2420. Nitrogen porosity was determined in accordance with DIN 66134. Before the measurement, the sample was degassed under reduced pressure at 200° C. for 16 h.

METHOD 2: MERCURY POROSIMETRY

Mercury porosimetry on the alpha-alumina shaped catalyst support bodies was conducted by means of an AutoPore V 9600 mercury porosimeter from Micrometrics (contact angle 140 degrees, Hg surface tension 485 dyn/cm, maximum head pressure 61000 psia). Mercury porosity was determined in accordance with DIN 66133.

The samples were dried at 110° C. for 2 h and degassed under reduced pressure before the analysis, in order to remove physically adsorbed species, for example moisture, from the sample surface.

METHOD 3: LOOSE BULK DENSITY OF POWDERS

In order to determine loose bulk density, the transition alumina or the alumina hydrate was introduced into a measuring cylinder via a funnel, ensuring that the measuring cylinder was not moved or agitated. The volume and weight of the transition alumina or of the alumina hydrate were determined. Loose bulk density was determined by dividing the volume in milliliters by the weight in grams.

METHOD 4: BET SURFACE AREA

BET surface area was determined to DIN ISO 9277 by means of nitrogen physisorption at 77 K. The surface area was ascertained from a 5-point BET graph. Before the measurement, the sample was degassed under reduced pressure at 200° C. for 16 h. In the case of the shaped alpha-alumina support bodies, more than 4 g of the sample was used on account of the relatively low BET surface area.

METHOD 5: DENSITY OF A POROUS SHAPED CATALYST SUPPORT BODY IN THE PACKED TUBE

Density in the packed tube was determined by introducing an amount of x g of shaped support bodies into a cylindrical glass tube having an internal diameter of 39 mm up to a mark indicating an internal tube volume of y mL. The glass tube was placed on a balance, and the increase in weight as a result of the support introduced was determined as x. The density in g/L was calculated as (x/y) x 1000.

METHOD 6: ANALYSIS OF THE TOTAL AMOUNT OF CA, MG, SI, FE, K AND NA CONTENTS IN ALPHA-ALUMINA SUPPORTS 6A. Sample Preparation for the Measurement of Ca, Mg, Si and Fe

About 100 to 200 mg (with an error of ±0.1 mg) of a support sample was weighed into a platinum crucible. 1.0 g of lithium metaborate (LiBO₂) was added. The mixture was melted in an automatic melting device with a temperature ramp up to max. 1150° C.

After cooling, the melt was dissolved by cautious heating in deionized water. Subsequently, 10 mL of semiconcentrated hydrochloric acid (concentrated HCl diluted with deionized water, volume ratio 1:1, corresponding to about 6 M) was added. Finally, the solution was made up to a volume of 100 mL with deionized water.

6B. Measurement of Ca, Mg, Si and Fe

The contents of Ca, Mg, Si and Fe were determined from the solution described in point 6A by means of inductively coupled plasma optical emission spectroscopy (ICP-OES) by means of a Varian Vista Pro ICP-OES.

Parameters Wavelengths [nm]: Ca 317.933 Mg 285.213 Si 251.611 Fe 238.204 Integration time: 10 s Atomizer: conical 3 mL Atomizer pressure: 270 kPa Pump rate: 30 rpm Calibration: external (matrix-adapted standards)

6C. Sample Preparation for the Measurement of K and Na

About 100 to 200 mg (with an error of ±0.1 mg) of a support sample was weighed into a platinum dish. 10 mL of a mixture of aqueous concentrated H₂SO₄ (95% to 98%) and deionized water (volume ratio 1:4) and 10 mL of aqueous hydrofluoric acid (40%) were added. The platinum dish was placed onto a sandbath and evaporated to dryness. After the platinum dish had been cooled, the residue was dissolved by cautious heating in deionized water. Subsequently, 5 mL of semiconcentrated hydrochloric acid (concentrated HCl diluted with deionized water, volume ratio 1:1, corresponding to about 6 M) was added. Finally, the solution was made up to a volume of 50 mL with deionized water.

6D. Measurement of K and Na

The amounts of K and Na were determined from the solution described in point 6C by flame atomic absorption spectroscopy (F-AAS) by means of a Shimadzu AA-7000 F-AAS.

Parameters: Wavelengths [nm]: K 766.5 Na 589.0 Gas: air/acetylene Gap width: 0.7 nm (K) / 0.2 nm (Na) Atomizer pressure: 270 kPa Calibration: external (matrix-adapted standards) Method 7: Tomographic focused ion beam scanning electron microscopy (FIB-SEM) and tomogram analysis

Focused ion beam scanning electron microscopy (FIB-SEM) was used to determine porosity and the pore topology properties of the materials. Experimentally, trilobal or tetralobal cylinder bodies of the materials were cut into small aliquots (about 2 mm) with a razor blade. The smaller material fragments were infiltrated and embedded with a low-viscosity epoxy resin (Spurr, Merck) using a pyramid-tip ultramicrotomy mold, and the resin was cured at 343 K for 12 hours. The resin-embedded sample block was then cut to size and polished in an ultramicrotome (Reichert Ultracut) with a diamond blade (Diatome) and positioned on an SEM probe with an epoxy adhesive.

Next, an electrically conductive layer was applied to the resin-embedded sample and the metal stump, using a colloidal graphite suspension in isopropanol. The sample mounted on the probe was then coated by sputtering with an about 20 nm-thick carbon layer in a BAL-TEC SCD 005 coater in order to achieve fully conductive connections and to minimize local charging artefacts during SEM imaging. FIB-SEM experiments were conducted in a Zeiss Auriga dual-beam microscope equipped with a Ga ion cannon and secondary electron and backscattering electron detectors.

For minimization of curtaining effects, with the aid of the ion cannon and metal gas injection devices of the dual-beam microscope, an additional layer of metallic Pt (about 30 nm) was applied to the region of interest (ROl). Anterior and lateral channels (depth about 45 µm) were machined with the focused beam of the Ga⁺ ions around the ROl in order to expose a finely polished anterior area (width about 30-40 µm x height 35-45 µm) of the block to be imaged. A cross-shaped fiducial marker was scratched on the upper surface of the sample alongside one of the lateral channels in order to serve as reference for the automatic drift correction during the FIB-SEM imaging. Next, an automatic slice & view algorithm was started in order to machine thin slices having a nominal thickness of 35-45 nm from the front face of the imaging block with the FIB cannon that was operated with an intensity of 2 nA and to record an SEM microphotograph of every newly exposed cross section with a secondary electron detector, while the electron cannon was operated at an acceleration voltage of 1-3 kV.

The collection of SEM microphotographs (2048 x 1536 pixels) was then processed by a vertical dilation correction for compensation of the angular alignment of the machining and imaging cannons in a dual-beam microscope (which form an angle of 54 degrees), and a bandpass FFT filter (see, for example, Kim, D., et al. (2019), Microscopy and Microanalysis, 25(5), 1139-1154) implemented in the FlJl-lmageJ 1.53 software was employed in order to reduce curtaining artefacts. The stack of microphotographs was aligned by means of a cross-correlation algorithm (see, for example, Yaniv Z. (2008) Rigid Registration. In: Peters T., Cleary K. (eds), Image-Guided Interventions. Springer, Boston, MA.). Finally, the stack was cut to a cubic data volume. The resulting reconstructed cubic tomograms had a volume of (20 to 23 µm)³ with elemental voxels of dimensions (15 to 45 nm)³.

For quantification of the material topology, voxels in the reconstructed FIB-SEM tomograms were classified in accordance with their contrast (grayscale) and assigned to various subvolumes or phases, i.e. pores, alumina and supported metal, by segmentation by means of a marker-based 3D watershed algorithm (E. Dougherty, editor, Mathematical morphology in image processing, chapter 12, pages 433-481. Marcel Dekker, 1993), implemented in Avizo® (ThermoScientific), followed by a fine adjustment of the automatically recognized volumes via controlled erosion expansion functions in order to remove artefact material “islands”, and a manual threshold adjustment in order to correct local grayscale gradients that result either from curtaining effects or shadowing phenomena.

The total porosity was then determined as the proportion of the total voxels corresponding to the subvolume pores. The average geometric pore tortuosity was determined as the ratio between geodesic and euclidean pore distances by propagating a centroid path algorithm through the pores of the tomograms of the subvolume (Gostovic, D. et al., Journal of the American Ceramic Society (2011) 94: 620-627). Four quadrants of equal size and the entire volume were considered separately for two independent tomograms per sample, in order to assess statistical uncertainty associated with the average geometric pore tortuosity.

For the analysis of pore constriction, an algorithm that combines a chamfer distance transformation, a 3D watershed operation and a numerical reconstruction, as implemented in Avizo® 2020.1-XPore (ThermoScientific) (E. Bretagne (2018) Mineralogical Limitations for X-Ray Tomography of Crystalline Cumulate Rocks, Durham University), was applied to the subvolume pores in order to produce a label field that separates individual pores (local broader sections of the 3D subvolume pores connected via narrower segments or necks). The algorithm was adjusted such that voxels considered to be connected are those that have at least one common vertex, and the contrast factor marker of the H maxima was set to 2. As a result, a pore network model was constructed, i.e. a 3-D model of spherical pores that are connected by cylindrical necks that correspond to the real pore space network. The constriction parameter was defined as the square of the ratio between the average diameter for all necks and the average diameter for all pores in the pore network model.

For silver-laden catalysts, porosity, pore tortuosity, pore constriction and the effective diffusion parameters for the alumina support were quantified after the tomogram voxels that had been segmented as supported metal were first assigned to the collection of voxels corresponding to the subvolume pores, as a result of which the supported metal was mathematically removed from the surface area of the alumina support material.

Production of Porous Alpha-alumina Catalyst Supports

The properties of the transition aluminas and alumina hydrates that were used for production of porous alpha-alumina catalyst supports are listed in table 1. The transition aluminas and alumina hydrates were sourced from Sasol (Puralox® and Rural®).

Transition aluminas and alumina hydrate according to table 1 were mixed in order to obtain a powder mixture. Processing aids (Vaseline® from Unilever and glycerol from Sigma-Aldrich) and water were added to the powder mixture. Vivapur® MCC Spheres 200 (microcrystalline cellulose, from JRS Pharma) were added to the mixture. Subsequently, further water was added in order to obtain a formable precursor material. The total amounts of all components are listed in table 2.

The formable precursor material was mixed to homogeneity by means of a roller-based mixer (Mix-Muller) and then extruded with a ram extruder to give shaped bodies. The shaped bodies had the shape of a trilobe with four passages, as shown in FIG. 4 . The extrudates were dried at 110° C. overnight (about 16 h) and then subjected to heat treatment in a muffled furnace at 600° C. for 2 h and then at high temperature (1475° C. for support A, 1430° C. for support B) for 4 h. The heat treatment was effected under air.

The dimensions of the dried applied layers were ascertained with a caliper gauge. The diameter of the circumscribed circle of the cross section at right angles to the support height was 11.6 cm. The term “circumscribed circle” relates to the smallest circle that fully encloses the trilobal cross section. The diameter of the inscribed circle of the cross section at right angles to the support height was 5.3 cm. The term “inscribed circle” relates to the largest possible circle that can be drawn within the trilobal cross section. The central passage had a diameter of 1.92 cm. The three outer passages had a diameter of 1.46 cm.

The resultant shaped support bodies A and B had an alpha-alumina content of more than 98% by weight, and Na, K, Mg, Ca contents below 100 ppm. The Fe content in both supports was 200 ppm. The Si content in support A was 100 ppm. The Si content in support B was 200 ppm.

The two shaped support bodies A and B had a density in the packed tube of 550 g/L.

Table 3 shows the physical properties of the supports produced according to table 2. FIGS. 5 and 6 show logarithmic differential intrusion and cumulative intrusion relative to pore size (pore diameter) for the supports produced according to table 2.

TABLE 1 Transition alumina * Bulk density [g/L] Pore volume [mL/g] ** Median pore diameter value [nm] ** Puralox SCFa 140 650 0.57 10.0 Puralox TM 100/150 UF 150 0.88 18.4 Puralox TH 200/70 300 1.23 37.4 Alumina hydrates * Bulk density [g/L] Pore volume [mL/g] ** Median pore diameter value [nm] ** Pural SB1 680 0.55 8.4 Pural TH 200 340 1.20 37.6 * Puralox products are transition aluminas produced from Pural products, i.e. boehmite ** determined by nitrogen sorption

TABLE 2 Support Transition alumina Alumina hydrate Pore-forming materials Processing aid Liquid A Puralox TH 200/70 173 g Puralox TM 100/150 UF 93 g Pural TH 200 67 g Vivapur MCC Spheres 200 250 g Vaseline 8.5 g Glycerol 8.4 g Water 454 g B * Puralox SCFa 140 173 g Puralox TM 100/150 UF 93 g Pural SB1 67 g Vivapur MCC Spheres 200 250 g Vaseline 8.3 g Glycerol 8.3 g Water 479 g * comparative example

TABLE 3 Support BET surface area [m²/g] Pore volume [mL/g] Pore volume present in pores [mL/g] ** (proportion of total pore volume) < 0.1 µm 0.1-1 µm 1-10 µm 10-100 µm > 100 µm r_(pv) *** A 2.0 0.57 0 (0%) 0.40 (70.2%) 0.12 (21.1%) 0.04 (7.0%) 0.01 (1.8%) 0.30 B * 2.0 0.53 0 (0%) 0.22 (41.5%) 0.21 (39.6%) 0.09% (17.0%) 0.01 (1.9%) 0.95 * comparative example ** determined by mercury porosimetry *** r_(pv) = ratio of pore volume present in pores having a diameter in the range from more than 1 to 10 µm to pore volume present in pores having a diameter in the range from 0.1 to 1 µm

Production of Catalysts

Shaped catalyst bodies were produced by impregnating supports A and B with a silver impregnation solution. The catalyst compositions are shown in table 4 below. The silver contents are reported in percent, based on the total weight of the catalyst. The amounts of promoter are reported in parts per million (ppm), based on the total weight of the catalyst.

Table 4: Catalyst composition (Ag contents are reported in percent by weight of the overall catalyst; amounts of promoter are reported in ppm, based on the weight of the overall catalyst)

Catalyst Support Ag_(CAT) ^(∗) [% by wt.] Li_(CAT) [ppm] S_(CAT) [ppm] W_(CAT) [ppm] CS_(CAT) [ppm] Re_(CAT) [ppm] K_(ADD) ** [ppm] 1 A 27.7 450 35 615 1025 1270 85 2 *** B *** 27.7 450 35 615 1025 1270 85 * the amount of silver was calculated ** K_(ADD) means the amount of potassium applied during the impregnation, and does not comprise the amount of potassium present in the alumina support before the impregnation *** comparative example

1. Production of a Silver Complex Solution

A silver complex solution was produced according to preparation example 1 of WO 2019/154863 A1. The silver complex solution had a density of 1.529 g/mL, a silver content of 29.3% by weight and a potassium content of 90 ppm.

2. Production of Catalyst Intermediates

In each case 100.0 g of support A (intermediate 1.1) or 100.4 g of support B (intermediate 1.2) was introduced into a 2 L glass flask. The flask was connected to a rotary evaporator that was put under a vacuum pressure of 80 mbar. The rotary evaporator was set to rotate at 30 rpm. 76.55 g (intermediate 1.1) or 76.86 g (intermediate 1.2) of the silver complex solution produced in step 2.1 was added to support A (intermediate 1.1) or support B (intermediate 1.2) under a vacuum pressure of 80 mbar over the course of 15 min. After the silver complex solution had been added, rotary evaporation was continued under reduced pressure for a further 15 min. The impregnated support was then left in the apparatus at room temperature (about 25° C.) and atmospheric pressure for 1 h and mixed cautiously every 15 min.

The impregnated material was placed onto a mesh in 1 to 2 layers. The mesh was exposed to an air stream of 23 m³ (STP)/h, the gas stream having been preheated to a temperature of 305° C. The impregnated material was heated up to a temperature of 290° C. at a heating rate of about 30 K/min and then kept at 290° C. for 8 min in order to obtain Ag-containing intermediates according to table 5. The temperatures were measured by mounting three thermocouples at a distance of 1 mm below the mesh. Subsequently, the catalysts were cooled down to ambient temperature by removing the catalyst intermediates from the mesh with an industrial vacuum cleaner.

Table 5: Ag-containing catalyst intermediates (Ag contents are reported in percent by weight of the overall catalyst; amounts of promoter are reported in ppmw, based on the weight of the overall catalyst intermediate)

Intermediate Support Ag_(CAT) * [% by wt.] K_(ADD) ** [ppm] 1.1 A 18.3 56 1.2 B 18.3 56 * the amount of silver was calculated ** K_(ADD) means the amount of potassium added during the impregnation, and does not comprise the amount of potassium present in the alumina support before the impregnation

3. Production of Catalysts

120.5 g of the Ag-containing intermediate 1.1 or 122.2 g of the Ag-containing intermediate 1.2 as produced in step 2.2 was introduced in each case into a 2 L glass flask. The flask was connected to a rotary evaporator that was put under a vacuum pressure of 80 mbar. The rotary evaporator was set to rotate at 30 rpm. For catalyst 1, 53.80 g of the silver complex solution produced in step 2.1 was mixed with 2.16 g of promoter solution l, 2.80 g of promoter solution ll and 4.69 g of promoter solution III. For catalyst 2, 54.56 g of the silver complex solution produced in step 2.1 was mixed with 2.19 g of promoter solution l, 2.84 g of promoter solution ll and 4.76 g of promoter solution III.

Promoter solution I was obtained by dissolving lithium nitrate (Merck, 99.995%) and ammonium sulfate (Merck, 99.4%) in deionized water in order to achieve an Li content of 2.85% by weight and an S content of 0.22% by weight. Promoter solution II was obtained by dissolving tungstic acid (HC Starck, 99.99%) in deionized water and cesium hydroxide in water (HC Starck, 50.42%) in order to achieve a target Cs content of 5.0% by weight and a W content of 3.0% by weight. Promoter solution III was obtained by dissolving ammonium perrhenate (Buss & Buss Spezialmetalle GmbH, 99.9%) in deionized water in order to achieve an Re content of 3.7% by weight.

The combined impregnation solution of silver complex solution and promoter solutions I, II and III was stirred for 5 min. The combined impregnation solution was added to each of the silver-containing intermediates 1.1 or 1.2 under a vacuum pressure of 80 mbar over the course of 15 min. After the combined impregnation solution had been added, rotary evaporation was continued under reduced pressure for a further 15 min. The impregnated support was then left in the apparatus at room temperature (about 25° C.) and atmospheric pressure for 1 h and mixed cautiously every 15 min.

The impregnated material was placed onto a grid in 1 to 2 layers. A nitrogen stream of 23 m³ (STP)/h (oxygen content: < 20 ppm) was passed through the grid, the gas stream having been preheated to a temperature of 305° C. The impregnated materials were heated up to a temperature of 290° C. at a heating rate of about 30 K/min and then kept at 290° C. for 7 min in order to obtain catalysts according to table 4. The temperatures were measured by mounting three thermocouples at a distance of 1 mm below the grid. Subsequently, the catalysts were cooled down to ambient temperature by removing the catalyst bodies from the mesh with an industrial vacuum cleaner.

FIGS. 7 and 8 show representative scanning electron micrographs of a cross section that had been exposed by machining of catalysts 1 and 2 by means of a focused ion beam.

Catalyst Tests

An epoxidation reaction was conducted in a stainless steel test reactor in a vertical arrangement, having an internal diameter of 6 mm and a length of 2.2 m. The reactor was heated with hot oil that was present at a particular temperature in a heating mantle. All subsequent temperatures are based on the temperature of the hot oil. The reactor was charged with 9 g of inert steatite spheres (0.8 to 1.1 mm), onto which was packed 26.4 g of comminuted catalyst that had been sieved to a desired particle size of 1.0 to 1.6 mm, and onto that was packed a further 29 g of inert steatite spheres (0.8 to 1.1 mm). The inlet gas was introduced into the upper part of the reactor in a “once-through” mode of operation.

The catalysts were introduced into the reactor at a reactor temperature of 90° C. at a nitrogen flow rate of 130 L (STP)/h at a pressure of 1.5 bar absolute. Then the reactor temperature was increased to 210° C. at a heating rate of 50 K/h and the catalysts were kept in that state for 15 h. Then the nitrogen stream was replaced by a stream of 114 L (STP)/h of methane and 1.5 L (STP)/h of CO₂. The reactor pressure was set to 16 bar absolute. Then 30.4 L (STP)/h of ethylene and 0.8 L (STP)/h of a mixture of 500 ppm of ethylene chloride in methane were added. Subsequently, oxygen was fed in stepwise until a final flow rate of 6.1 L (STP)/h has been attained. At this juncture, the inlet composition consisted of 20% by volume of ethylene, 4% by volume of oxygen, 1% by volume of carbon dioxide and ethylene chloride (EC) moderation of 2.5 ppmv (parts per million, based on volume), using methane as balance at a total gas throughput of 152.8 L (STP)/h.

The reactor temperature was increased to 225° C. at a heating rate of 5 K/h and then to 240° C. at a heating rate of 2.5 K/h. The catalysts were kept under these conditions for 135 hours. Then the EC concentration was reduced to 2.2 ppmv and the temperature was lowered to 225° C. Then the inlet gas composition was altered stepwise to 35% by volume of ethylene, 7% by volume of oxygen, 1% by volume of carbon dioxide with methane as balance, and a total gas throughput of 147.9 L (STP)/h. The temperature was adjusted so as to attain an ethylene oxide (EO) concentration in the outlet gas of 3.05%. The EC concentration was adjusted to optimize the selectivity. The results of the catalyst tests are summarized in table 6.

TABLE 6 Cat. Support Time on stream *** [h] EO selectivity [%] Reactor temperature [°C] # Geometric tortuosity τ Effective diffusion parameter η ** 1 A 1.20 ± 0.02 0.075 ± 0.007 600 89.0 235 2 * B * 3.92 ± 0.16 0.006 ± 0.001 600 87.9 234 * comparative example ** effective diffusion parameter η, defined by equation 3 *** time on stream begins from the juncture of feeding of oxygen into the ethylene-containing feed stream

The error intervals correspond to the standard error SE, defined as SE = s/(N)^(½), where s is the standard deviation and N is the number of evaluations, in the case of parameter η propagated from the uncertainties of porosity, constriction and geometric tortuosity.

It is apparent that the catalyst 1 obtained from inventive support A has much higher selectivity than the catalyst 2 obtained from comparative support B. 

1-13. (canceled)
 14. A porous shaped catalyst support body comprising at least 85% by weight of alpha-alumina, wherein the support has a total pore volume in the range from 0.5 to 2.0 mL/g as determined by mercury porosimetry, and a pore structure characterized by - a geometric tortuosity τ in the range from 1.0 to 2.0; and - an effective diffusion parameter η in the range from 0.060 to 1.0; wherein geometric tortuosity τ and effective diffusion parameter η are determined by image analysis algorithms from computer-assisted 3D reconstructions of focused ion beam scanning electron microscope analyses.
 15. The porous shaped catalyst support body according to claim 14, having a density in the packed tube of more than 450 g/L.
 16. The porous shaped catalyst support body according to claim 14, having a BET surface area in the range from 0.5 to 5.0 m²/g.
 17. The porous shaped catalyst support body according to claim 14, obtained by i) providing a precursor material comprising, based on the content of inorganic solids, - at least 50% by weight of a transition alumina having a loose bulk density of not more than 600 g/L, a pore volume of at least 0.6 mL/g and a median pore diameter value of at least 15 nm; and - not more than 30% by weight of an alumina hydrate; ii) shaping the precursor material to shaped bodies; and iii) calcining the shaped bodies in order to obtain the porous shaped catalyst support body.
 18. The porous shaped catalyst support body according to claim 17, wherein the transition alumina has a loose bulk density in the range from 50 to 600 g/L and a pore volume of 0.6 to 2.0 mL/g.
 19. The porous shaped catalyst support body according to claim 17, wherein the transition alumina comprises a phase selected from gamma-alumina, delta-alumina and theta-alumina.
 20. The porous shaped catalyst support body according to claim 17, wherein the alumina hydrate comprises boehmite and/or pseudoboehmite.
 21. The porous shaped catalyst support body according to claim 17, wherein the precursor material also comprises pore-forming materials, lubricants, organic binders and/or inorganic binders.
 22. The porous shaped catalyst support body according to claim 17, wherein the precursor material is shaped to shaped bodies by extrusion, tableting, pelletization, casting, forming or microextrusion.
 23. The porous shaped catalyst support body according to claim 17, wherein the calcining is performed at a temperature of at least 1100° C.
 24. A shaped catalyst body for preparation of ethylene oxide by gas phase oxidation of ethylene, comprising at least 15% by weight of silver, based on the total weight of the catalyst, deposited on a porous shaped catalyst support body according to claim
 14. 25. A process for producing a shaped catalyst body according to claim 24, in which a) the porous shaped catalyst support body is impregnated with a silver impregnation solution; and the impregnated porous shaped catalyst support body is optionally subjected to drying; and b) the impregnated porous shaped catalyst support body is subjected to a heat treatment; wherein steps a) and b) are optionally repeated.
 26. A process for preparing ethylene oxide by gas phase oxidation of ethylene, comprising the reaction of ethylene and oxygen in the presence of a shaped catalyst body according to claim
 24. 